Buddha is not the avatar of vishu for one basic reason. The core teaching of buddhism, i mean the core teaching, the third turning of the wheel of dharma teaches that there is no such thing as aatma, where as in hinduism there is thing called aatma. Most people think buddhism as similar to other religion , but the fact is it is so completly different. Heart sutra is the most popular sutra in Buddhism and is the best known perfection of wisdom text. It explains the teaching of non-attachment, which is the doctrine of emptiness (sunyata). The translation of sunyata in Nepalese language is zero. The heart sutra makes it clear that empty means “empty of self (svabhava)(aatma).” The significance of heart sutra is that all things including material and non material things are empty. Secondly, form is not different from emptiness, and emptiness is not different from form. Form is emptiness and emptiness is form. Similarly, this can be said for perception, karma and consciousness. This means all the five skandhas are empty of svabhava. Form, feeling perceptions, karma and consciousness that describe reality in Buddhism, also called pratitya samutpada, is empty. This does not mean these things do not exist, it just means that the five skandhas are not fixed and changes constantly. They are impermanent and due to this they do not have svabhava. These skandhas are not ultimate and eternal, but are actually conditional. They arise and cease. They are constantly moving and changing. These don’t have anything fixed. They are empty. In fact the five skandhas are unreal and insubstantial because the reality we are in is created by our own minds based on form, feeling, perception and consciousness. Because everything in the world is without svabhava, there is nothing we can hold on to that is fixed. We need to let go of all the desires, aversions and ego etc. Furthermore, we need to realize, that all things have “no self” even the very truth of emptiness is empty. Everything is empty in the sense it does not have svabhava is the core of the heart sutra and Buddhism. If Buddha was a avatar of vishu, he was a very bad avatar because he is teaching completely opposite of what Hinduism is about.