Shri Gautama Buddha is only considered by the Buddhists as a separate leader. However even Buddhists if they study the Buddha, they realize that He was only trying to encourage the truth interpretation of the Vedas and its Arya Dharam. 
Buddhist scholar Rahula Vipola wrote that the Buddha was trying to shed the true meaning of the Vedas. Buddha is said to be a knower of the Veda (vedajña) or of the Vedanta (vedântajña) (Sa.myutta, i. 168) and (Sutta Nipâta, 463).
For all inquiries, please read this version of Buddha as an Avatar of Vishnu