Both वादरायण and बादरायण are in use. In Sanskrit Dictionary he is referred to as बादरायणः, who is often taken for another name of Vyas.

राहुल सांकृत्यायन अपनी पुस्तक दर्शन दिग्दर्शन में उन्हें शब्दप्रमाणक ब्रह्मवादी वादरायण (CE 300) के शीर्षक में व्याख्या करते हैं ।

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Note added at 13 hrs (2010-05-09 02:18:41 GMT)
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VedVyas and Badarayan are actually two different personalities. Ved Vyas, the author of Mahabharta, is said to have lived in the BC era--3rd or 2nd century BC. Badarayan lived around AD 300. Badarayan, as second Vyas, is purely a title of honor to him--meaning he was equal to Vyas. Hindus, however, date VedVyas to a far earlier period, which, however, is contadicted by Buddhist scriptures. Had Ved Vyas lived before Buddha, Buddha would have been aware of Mahabharata, and consequently, there would be refelection of Mahabharata in the earlier Buddhist scriptures as well.