Namaste Awaiting,
I will give you my opinion on the Buddha as he is understood by Hindus. I realize that Buddhists do not agree with the Hindu conception of Buddha, and I mean no disrespect.
First a bit of CR, as we are on a CR board

. While Hinduism and Buddhism agree on some central concepts including the doctrines of dharma, karma, reincarnation, and samsara, Hindus have historically disagreed with the Buddhist doctrines of Shunyata, Anatta, dukkha, nirvana, and, of course, in the rejection of the Vedas. As you know, Sanatana Dharma is a decidedly theistic religion and the concept of Brahman (Ultimate Reality) and Atman (Soul) are central to it. To Hindus, Brahman is all-encompassing, all-pervasive, the source and destination of every Atman, and the eternal existence of Brahman cannot be denied. The final state of moksha (oneness with Brahman) is described in the Vedic texts as a state of fullness or completeness (Purnam). Clearly, the concepts of Brahman, Atman and Purnam are in stark contrast with Anatta and Shunayata.
In Hinduism, Buddha refers only to the person, Siddhartha Gautama (as there is no concept of Buddhahood). The story of the Buddha Hindus know is the same. Hindus generally believe that Siddhartha Gautama was born into the Shaivite Hindu family (Shakya clan) and lived his life as a good Hindu! The Buddha is a well-known and respected personality, and is considered a great sage (Shakya Muni), who attained self-realization by his own meditative and ascetic efforts. Hindus believe that the Buddha came to the same realization as did the sages before him who discovered the eternal principles of the Vedas and Upanishads. Some modern Hindu masters have even said that the Buddha expounded the same Vedic truths using different terms, which were misconstrued by his followers who founded Buddhism (obviously, Buddhists would strongly disagree!

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In the Vaishnavite denomination of Sanatana Dharma, Brahman is personified as Lord Vishnu, who periodically incarnates on Earth. Some of Lord Vishnu’s incarnations are considered primary (like Rama and Krishna), while others are considered secondary. Only the primary incarnations are worshipped by Vaishnavite Hindus. The authority on the incarnation doctrine in Vaishnavism is the Bhagavat Purana, which mentions 24 incarnations of Vishnu, one of which is Buddha. Vaishnavites regard the Buddha as the most recent incarnation of Lord Vishnu, but being a secondary incarnation, Buddha is not worshipped in Hindu temples (at least to my knowledge). According to Vaishnavism, the Buddha avatar came to reinstate dharma in the present age, to spread compassion among humanity, and to stop the prevalent adharma of sacrificing and consuming animals, especially as a ritual practice. This idea is accepted in many branches of Vaishnavism, but not all. In some branches, Buddha is not accepted as an incarnation of Lord Vishnu mainly because the Alvar Vaishnavite saints, who lived between 600-900 CE, spoke out against Buddhism in their devotional hymns.
I don’t know whether Buddha was accepted as an incarnation by Hindus since the early days. There is a theory out there that says that the idea that Buddha was an incarnation of Vishnu became popular about a thousand years after the life of the Buddha, and this eventually entered into the Bhagavat Purana. Still others say the deification of Buddha was a concerted effort among Hindus to absorb Buddhism. I don’t know how far any of these claims are true.
With Regards.