Hindu roots of Vesak Festivals?

[Dave Turpie]

Hello list members,
        Here's hoping someone in the know will indulge a query that balances
precariously on the edge of being inter-disciplinary. I am currently
researching early forms of the Theravadan Buddhist ritual known as Vesak
(derived from vaisakha, the second month in the Indian lunar calendar).
Given that in some circles, Gotama Buddha is considered to be the ninth
incarnation of Visnu, and the month of Vaiskha is generally associated with
Visnu celebrations, there would seem to be a link between this buddhist
festival which celebrates the birth, death, and enlightenment of the buddha
(all ocurring on the same full moon day in this month)and earlier (or
current)Visnu festivals of some sort.
        At the moment this all pure speculation with no textual basis to support
such claims. Can anyone offer some guidance in the form of text or an
actual festival that might intimate at a connection between Vesak and a
Visnu festival either held in the past or currently being sponsored?

Any help would be greatly appreciated, David T.