The Mahāvastu, the ‘Great Story,’ claims to be a vinaya, a word meaning ‘discipline’ and usually used for a collection of rules for monks or nuns. In fact, it is actually a long and rather chaotic biography of the Buddha compiled from oral and written sources between about the 2nd century BCE and the 3rd century CE. A veritable treasure house of legends, ballads, authentic historical information, devotional poetry and Dhamma teachings, it also contains numerous Jātaka stories, some from the Tipiṭaka, many others unique to it.
Le Mahāvastu, Sanskrit text, was published for the first time with introduction by E. Senart with a detailed conspectus of contents in the Introduction, Paris 1882-1897. A. Barth in Revue de l'Histoire des Religions., 11, 1885, p. 160; 42, 1900, p. 51 and Journal des Savants 1899, p. 459, p. 517, p. 623. E. Windisch, the Composition of the Mahāvastu, Leipzig 1909. A conspectus of the contents is also given by Rajendralal Mitra in his Nepalese Buddhist Literature, pp. 113-161.
The book gives itself the title of: “The Vinayapiṭaka according to the text of the Lokottaravādis belonging to the Mahāsaṅghikas.” These Mahāsaṅghikas, that is, the adherents of the Mahāsaṅgha or the Great Order are according to concurrent reports the most ancient Buddhist schismatics.
“Nothing in the perfectly Awakened Ones is comparable to anything in the world but everything connected with the great ṣis is exalted above the world.” They wash their feet although no dust attaches to them, they sit under the shade although the heat of the sun does not oppress them, they take nourishment although they are never troubled with hunger, they use medicine although they have no diseases (Windisch loc. cit. p. 470). According to  the Mahāvastu, the Lokottaravādis belong to the Madhyadeśa or the 16 countries lying between the Himālaya and the Vindhya mountains (Mahāvastu V.1, p. 198.)
Entirely in keeping with this doctrine, the biography of the Buddha which forms the principal contents of the Mahāvastu is related as an “Avadāna” or a miraculous history. It is clearly not thereby differentiated much from the texts of the Pāḷi canon which are devoted to the life of the Buddha. Here in this Sanskrit text just as in the Pāḷi counterpart we hear of miracles which accompanied the conception, the birth, the illumination, and the first conversions brought about by the Buddha.
The Mahāvastu harmonizes with the Pāḷi Nidānakathā in this that it treats of the life of Buddha in three sections, of which the first starts with the life of the Bodhisattva in the time of the Buddha Dīpaṅkara (V. 1, 193) and describes his life in the time of other and earlier Buddhas. The second section (in V. 2, 1) takes us to the heaven of the Tuṣita gods, where the Bodhisattva who is re-born there is determined to seek another birth in the womb of Queen Māyā and relates the miracle of conception and the birth of the prince, of his leaving the home, his conflict with Māra, and the illumination which he succeeds in acquiring under the Bodhi Tree. The third section (V. 3), lastly recounts, in harmony with the principal features of the Mahāvagga of the Vinayapiṭaka, the history of the first conversions and the rise of the monastic order. And this is also one reason why the Mahāvastu is described as belonging to the Vinayapiṭaka, although barring a few remarks on the initiation of the Order it contains next to nothing about the Vinaya proper or the rules of the Order.
Note: The Mahāvastu does not contain the Pāḷi technical expressions, Dūrenidāna, Avidūrenidāna and Santikenidāna [which are found in the late Jātakanidāna]. See Windisch loc. cit. p. 473, 476 ff. 
When we, however, say that the Mahāvastu recounts the main outline of the life of the Buddha for the Lokottaravādis, that by no means implies that this exhausts the contents of the work; nor does it give an adequate idea of its composition. Far from being a literary work of art, the Mahāvastu is rather a labyrinth in which we can only with an effort discover the thread of a coherent account of the life of the Buddha. This account is constantly interrupted by other material, specially by the numerous Jātakas and Avadānas and also by dogmatic Sūtras. We find no order. Sometimes an attempt is made to put together in a loose fashion the various component parts of the work. Moreover, the same story is frequently repeated whether it be an episode in the life of the Buddha or a Jātaka, being related twice one after another, first in prose and then in verse, although in a more or less diverging version. But in several passages the same episodes recur with a trifling difference. Thus the legend of the Buddha's birth is recounted no less than four times (Windisch, Buddha's Birth, p. 106, 124 ff.). Again language is also not uniform. No doubt the whole work, both the prose and verse, is written in what we call “mixed Sanskrit,” but this dialect makes a varying approach to Sanskrit. The more disparate it is from Sanskrit, the more ancient it appears (Oldenberg Zeitschift der Deutschen Morgenlandischen Gesellschaft 52, 663).
Importance of Mahāvastu
Despite this and not withstanding the circumstance that out of this book we learn hardly anything new on the life of the Buddha or of the Lokottaravādis, it is [still] of the greatest importance because it preserves for us many ancient traditions and old versions of texts which also occur in the Pāḷi canon. Thus the setting out of his home by the Prince Siddhārtha, the celebrated abhiniṣkramaṇa of Sanskrit books, is related, as in the Pāḷi Majjhimanikāya (26 and 36) in the most archaic fashion (V. 2, 117).
As  an instance of the various strata of the book we may mention another version of the same episode in the life of the Buddha and belonging to a later period which follows immediately after the first and more ancient recital in Mahāvastu. Similarly we find early versions of the celebrated “Benares sermon” and presentments of the following well-known texts in the Pāḷi canon:- The Mahāgovinda Sutta (Dīghanikāya 19) the Dīghanakhasutta (Majjhimanikāya, 74) the Sahassavagga of the Dhammapada, the Khuddakapāṭha, the Pabajjā, the Padhāna and the Khaggavisāṇa Suttas belonging to the Suttanipāta, and pieces from Vimānavatthu and the Buddhavaṁsa (Oldenberg Zeitschift der Deutschen Morgenlandischen Gesellschaft 52, 659 f. 665 f. Windisch Māra and Buddha, 316 f, 322 f). There are poems, moreover, on the birth of the Buddha and vestiges of ancient Buddhistic ballads which we so often come across.
Quite of special value is, however, the Mahāvastu as a mine of Jātakas and other stories. These have been separately treated by Serge d'Oldenberg (Journal of the Royal Asiatic Society 1896, p. 335 f.) and by Barth (Journal des Savants 1889, p. 625 f.) Charpentier has discussed a few of the Jātakas in the Mahāvastu in his history of the Pacceka Buddhas (p. 2 f. 12 f, 25 f.) A good half of the book consists of Jātakas which are related partly in prose with verses inserted, or first in prose and then again in verse. Further we see the Bodhisattva now as a universal sovereign, now as the son of a merchant, then as a Brahman, again as a Nāga prince, as a lion, as an elephant, etc. Many of the Jātakas are versions of the same story which we find in the Pāḷi book of Jātakas. They harmonize word for word with the Pāḷi and many a time show more or less divergence. Thus, for instance, the Śyāmakajātaka (V. 2, p. 209 f.), the pathetic story of the Brahman's son who is shot dead with his arrow by King Peliyakṣa is only a  version of the Sāmajātaka (Pāḷi No. 540] so well known to us. The Kinnarījātaka (V. 2, p. 94 f.) corresponds in character, though not in contents to the Kinnara legend in the Jātaka book. Kuśajātaka appears once (V. 2, p. 420 f.) in a recension which is tolerably divergent from Pāḷi, a second time (V. 1, p. 3 f.) in metrical form which betrays resemblances with the Pāḷi gāthās. The story of Amara, the smith's daughter, (V. 2, p. 836) answers to the Pāḷi Jātaka No. 387. The Markatajātaka (V. 2, p. 246 f.) is the fable of the monkey and the crocodile and is known to us as No. 208 of the Pāḷi Jātaka book. The history of Nāliṇī who is seduced by Eka Śṅga, grows into a highly developed legend in Mahāvastu (V. 3, p. 143 f.). But it retains some of the more ancient features which have disappeared in the prose Pāḷi Jātaka of Isisiṅga (Luders, Nachrichten von der K. Gesellschaft der Wissenschaften zu Gottingen 1901, p. 20 f.)
There are, however, many Jātakas and Avadānas in the Mahāvastu which have nothing corresponding to them in Pāḷi. In these are especially glorified again and again the extraordinary propensity to self-sacrifice and generosity on part of the Bodhisattva. Thus as King Arka, for example, the Bodhisattva bestows upon the Buddha of the age 80,000 grottoes or cave temples fashioned out of the seven kinds of precious stones (1, 54). On another occasion he surrenders his wife and child only [in order] to learn a wise maxim (1, 91 f.) As a beggar he is more pious than King Kki, for he kills no living being and places his pots on crossways in order that they may be filled with rice and grain for the hungry; and when he hears that his parents in his absence have given away to the Buddha the straw with which he had shortly before embellished his hut he rejoices over it for a month (1,317 f.) [Despite what Nariman said above this last corresponds to Ghaṭikāra the potter's story in Majjhimanikāya 81]. 
Many of the narratives bear the impress of a Brahmanic or Purāṇic character. Such is, for instance, the history of Brahmadatta who is childless and betakes himself to the ṣis upon which three birds are borne to him which speak with a human voice and utter many sapient proverbs. This story reminds us of the beginning of the Mārkaṇḍeya Purāṇa. And incidentally it may be observed that portrayal of hell in the beginning of the Mahāvastu has points of contact with the same Purāṇa. It is, however in the Pāḷi tradition that we find the foundation of the visit of Maudgalyayāna to the 8th Inferno as well as his sojourn in the world of beasts and the world of Pretas, the Asūras, and various kinds of deities. For in the Pāḷi tradition also Moggallāna is a saint who roams through heaven and hell and all the worlds. However, the Rājavaṁśa or the History of the Kings to whose dynasty Śākyamuni belonged begins entirely after the fashion of the Purāṇas with an account of the creation (1, 338 ff.) The sprit of the Purāṇas is also breathed by the Jātaka (1, 283 ff.), in which a ṣi named Rakṣita who is the Bodhisattva, attains to such miraculous powers as an ascetic that he touches the sun and the moon with his hand. The spirit of the Purāṇas is very similar to that of the Mahāyāna and many of the stories in the Mahāvastu betray the same partiality for the phantasmagorial - astounding sorcerers to perform the miracles of saints, so peculiar to the Mahāyāna texts. To this class belongs “the Story of the Umbrella” (Chattravastu I, 253 ff.) After the Buddha had freed the city of Śrāvastī of a terrible plague caused by Yakṣas, gods or spirits hold up umbrellas over the Buddha to do him honour. The latter however with his usual compassionateness makes one Buddha to appear under each umbrella by virtue of his supernatural powers so that each god believes that the Buddha is seated under his own umbrella. 
More Mahāyāna Affinities
And, although the Mahāvastu belongs to the Hīnayāna and has contacts with much which may or actually does occur in the Pāḷi texts of the Theravādis, it embodies a good deal which makes an approach to the Mahāyāna. Thus, for instance, we find in the first volume (1, 63-193) a large section on the ten Bhūmis or places which a Bodhisattva has to go through and the description of the virtues which he must possess in each of the ten stages. In this section has been interpolated a Buddhānusmti (1, 163 ff.) that is, a hymn to the Buddha who in no way is here different from Viṣṇu or Śiva in the stotras of the Purāṇas. It is also in keeping with the idea of the Mahāyāna when it is said that the power of Buddha is so great that the adoration of the Exalted One alone suffices for the attainment of Nirvāṇa (II, 362 ff.) and that one earns for oneself infinite merit when one only circumambulates a stūpa and offers worship with flowers and so forth. That from the smile of the Buddha proceed rays which illuminate the whole Buddha field (Buddhakṣetra) occurs innumerable times in the Mahāyāna texts (III, 137 ff). It is also a Mahāyānist conception when mention is made of a great number of Buddhas and when it is stated that the Bodhisattva is not generated by father and mother, but springs directly from his own properties (Windisch, The Buddha's Birth, p. 97 Note, p. 100 f. and p. 193 f.)
Antiquity of Mahāvastu
The nature of the composition of the Mahāvastu entails the difficulty that the period when it was composed is very hard to determine. Many circumstances point to a high antiquity, for instance, the fact that it belongs to the Lokottaravāda school and also its language. That the work is entirely written in “mixed Sanskrit” while in the Mahāyāna texts this dialect alternates with Sanskrit, is a mark  of its greater antiquity. For, as Barth said Sanskrit is in Buddhist texts only an interloper (Journal des Savants, 1899,p. 459).
Certainly old are those numerous pieces which the Mahāvastu has in common with the Pāḷi canon and which go back to ancient Pāḷi sources. The gāthās of the Khadgaviśāṇa Sūtra (I. 357,) may be even older than the corresponding Khaggavisāṇa Sutta in the Pāḷi Suttanipāta. When, however, in the Mahāvastu these verses are sung by five hundred dying Pratyeka Buddhas then in their mouth they refrain. “He wanders lonely like a unicorn” sounds peculiarly incongruous and it becomes improbable that the prose portion should be as old as the gāthās.
To the time of the first century after Christ likewise point the Mahāyānist features already indicated as well as a few passages which seem to have been influenced by the sculptors of the Gandhāra art. When for example, in the scene of the flower miracle, the lotus flowers in the form of a circle fall round the halo of the Buddha, it may be noted that the halo was first introduced into India by Greek artists (see A Foucher Journale Asiatique 1903, p. 10, part II, p. 208, and his L'art grecobouddhique du Gandhāra, vol. I, p. 622; besides, the many Buddhas under the umbrellas remind us of the sculptured monuments). The reference in the Mahāvastu to the Yogācāras brings us down to the fourth century (I, 120); and so do the allusions to the Huns and the most interesting ones to the Chinese language and writing and the characterisation of astrologers as “Horāpāṭhaka” (III, 178). But the core of the Mahāvastu is old and probably was composed already two centuries before Christ, although it has been expanded in the fourth century after Christ and perhaps even at a later period. For it is only the embellishment that has been borrowed from the Mahāyāna, while on the other hand, it is merely a feeble admixture of the Mahāyāna doctrine proper and not of the Mahāyāna mythology which we find in the Mahāvastu.