A closer look into early texts shows the idea that Vishnu and Buddha are one is a myth:

In the Maha-samya Sutta there was an occasion when the devas from almost all the planes came to see the Buddha when he was dwelling in the Great Wood together with 500 bhikkhus, all of them arahants. The Buddha introduced their names to the monks, Vishnu was one of those present. The Buddha mentioned him by the name Venhu.

The Venhu Sutta shows Vishnu as one of the young devas who came to visit and talked with the Buddha:

At Savatthi. Standing to one side, the young deva Venhu recited this verse in the presence of the Blessed One: " Happy indeed are those human beings attending on the Fortunate One. Applying themselves to Gotama's Teaching, who train in it with diligence."
 
The Blessed One said: "When the course of teaching is proclaimed by me, O Venhu," said the Blessed One, "Those meditators who train therein. Being diligent at the proper time. Will not come under Death's control."-  The Connected Discourse of the Buddha"  A Translation of the Samyutta Nikaya by Bhikkhu Bodhi ,page 432)
 
According to "Hinduism and Buddhism An Historical Sketch" (Vol. 2 page 746) , Sir Charles Elliot who was a British diplomat mentioned that this correlates with the Rig Veda text before Hinduism started. Both texts mentioned that Vishnu and Shiva are minor deities originally.

" Vishnu and Rudra (Shiva) are known even to the Rig Veda but as deities of no special eminence. It is only after the Vedic age that they became , each for his own worshippers, undisputed Lords of the Universe…..The Pali Pitakas frequently introduce popular deities , but give no prominence to Vishnu and Siva. They are apparently mentioned under the names of Venhu and Isana, but are not differentiated from a host of spirits now forgotten. ….The suttas of the Digha Nikaya in which these lists of deities occur were perhaps composed before 300 B.C. "- Sir Charles Elliot