„In every third world age (Dvapara), Vishnu, in the person of Vyasa, in order to promote the good of mankind, divides the Veda, which is properly but one, into many portions. Observing the limited perseverance, energy, and application of mortals, he makes the Veda fourfold, to adapt it to their capacities; and the bodily form which he assumes, in order to effect that classification, is known by the name of Veda-vyasa. Of the different Vyasas in the present Manvantara and the branches which they have taught, you shall have an account. Twenty-eight times have the Vedas been arranged by the great Rishis in the Vaivasvata Manvantara... and consequently eight and twenty Vyasas have passed away; by whom, in the respective periods, the Veda has been divided into four. The first... distribution was made by Svayambhu (Brahma) himself; in the second, the arranger of the Veda (Vyasa) was Prajapati... (and so on up to twenty-eight).“
Vishnu Purana (Book 3, Ch 3), in The Vishńu Puráńa: A System of Hindu Mythology and Tradition http://books.google.co.in/books?id=bkEpAAAAYAAJ, p. 219.