Modern Buddha and Vishnu Avatar Buddha are different

The Vedas encode instructions according to the eligibility or qualification of various living beings, especially human beings. But in the course of time, ignorant men took the tamasika orders to be the only instruction of the Vedas and engaged in the extensive killing of animals, sometimes even sacrificing human beings during worship of the demigods. At that time, the Supreme Lord descended in the form of Buddha and outwardly rejected the teachings of the Vedas for the welfare of human beings incapable of comprehending the true teachings of the Vedas. This implies that He disputed and cancelled His own prior teachings, propounded the futility of belief in God and preached to human beings four noble truths, to free them from their violent practices. This act of Buddha provided instantaneous benediction to mankind of that period. As Lord Buddha was the Supreme Lord Himself, many people resolved to follow ahimsa-dharma—the path of non-violence, due to His influence. As a result of non-violence, the hearts of human beings became pious and their qualifications gradually increased, so Lord Siva appeared as Sankaracarya. He re-established the supreme authenticity and decorum of the Vedas, and founded the philosophy of ‘brahmakarana-vada’ (Brahman as ultimate cause). In later ages, the Vaishnava stalwarts built the philosophy of bhakti upon this same foundation stone. From the personal and aggregate point of view, these are the steps of progress.

Srila B.B. Tirtha Maharaja