I had seen the arguments of some of the Vaishnavas before, that Gautama rejected the Vedic precepts to keep the evil and ignorant out of the way of Dharma, but after all, why would God send someone who rejected him countless times? Someone that would found a new line of thoughts that for a time usurped the Vedic faith in India? Is there anything that can prove this in our scriptures?
All serious Vaishnavas consider Buddha an avatar of Vishnu, but the question is which Buddha? First, let us prove that the Buddha was indeed an avatar.
Some claim that the Buddha was either an interpolation in the puranas or that the puranas themselves came after the time of the Buddha. Allegedly, this would bring Buddhists into the fold of Hinduism. However, that would make the following verse of Srimad Bhagavatam historically inaccurate.
When the atheists, after being well versed in the Vedic scientific knowledge, annihilate inhabitants of different planets, flying unseen in the sky on well-built rockets prepared by the great scientist Maya, the Lord will bewilder their minds by dressing Himself attractively as Buddha and will preach on subreligious principles. [SB 2.7.37]
Obviously, there were no rockets toward the beginning of the Kali Yuga (traditionally accepted as 3102 BCE but may actually be debatable as seen here), so this verse would not further the puranic "agenda" to subsume Buddhism under Hinduism. Actually, this verse is referring to the Buddha in another kali-yuga as Srila Prabhupada's commentary says.
According to Śrīla Jīva Gosvāmī, the Buddha incarnation mentioned in this verse appeared in a different Kali age. In the duration of life of one Manu there are more than seventy-two Kali-yugas, and in one of them the particular type of Buddha mentioned here would appear.
Part of the actual confusion regarding the Buddha avatar stems from the fact that there are different Buddhas mentioned in the puranas (the above verse being an example). One chapter of the Vishnu Purana also discusses Buddha being sent by Vishnu to delude the atheists, but that Buddha seemed to have emanated the Jain doctrine as well. (I do not currently have the relevant verses.) With that out of the way, let us turn to the Buddha of the present Kali Yuga.
Then, in the beginning of Kali-yuga, the Lord will appear as Lord Buddha, the son of Añjanā, in the province of Gayā, just for the purpose of deluding those who are envious of the faithful theist. [SB 1.3.24]
Based on my research, I have come across three different views regarding the identity of the avatara known as Buddha.
1. The Buddha Mentioned Throughout the Puranas Is Gautama Buddha.
According to Prabhupada's commentary on verse 1.3.24, Gautama Buddha incarnated to stop animal sacrifice since it was being used as an excuse to eat meat (which is typical in this age).
Because the asuras or the so-called scholars of Vedic literatures put forward the evidence of animal-killing in the Vedas, Lord Buddha superficially denied the authority of the Vedas. This rejection of the Vedas by Lord Buddha was adopted in order to save people from the vice of animal-killing as well as to save the poor animals from the slaughtering process of their big brothers who clamor for universal brotherhood, peace, justice and equity. There is no justice when there is animal-killing. Lord Buddha wanted to stop it completely, and therefore his cult of ahiṁsā was propagated not only in India but also outside the country.
As for why the Buddha would go against the Vedas, Prabhupada says that it was due to the circumstances of that time.
Technically Lord Buddha’s philosophy is called atheistic because there is no acceptance of the Supreme Lord and because that system of philosophy denied the authority of the Vedas. But that is an act of camouflage by the Lord. Lord Buddha is the incarnation of Godhead. As such, he is the original propounder of Vedic knowledge. He therefore cannot reject Vedic philosophy. But he rejected it outwardly because the sura-dviṣa, or the demons who are always envious of the devotees of Godhead, try to support cow-killing or animal-killing from the pages of the Vedas, and this is now being done by the modernized sannyāsīs.
2. Gautama Buddha Is Not the Avatar of Vishnu
This article by Stephen Knapp argues that the avatar of Vishnu known as Buddha incarnated before Gautama Buddha. The former is known as the son of Añjanā while the latter was the son of Shuddhodana and Mayadevi. (Someone could claim that Añjanā was an epithet of Mayadevi, the mother, but I do not think she was ever called Añjanā in her life, nor did she have a name change.) The former was born in Kikata, also known as Gaya, while the latter was born in Kapilavastu, Nepal.
Knapp discusses several verses from Srimad Bhagavatam, Buddhist scriptures, and the Amarakosha to prove his point. I will just quote what he said about the Amarakosha.
In these verses, starting with sarvajnah and finishing with munih are eighteen names addressing the original Vishnu incarnation Lord Buddha. The next seven names beginning with Shakya-munistu to Mayadevi-Sutascha refer to Shakya Simha Buddha. The Buddha referred to in the first eighteen names and the Buddha referred to in the later seven names are clearly not the same person. [This clearly indicates that knowledge of the two Buddhas was well known long ago.] In the commentary on Amarakosha by the learned Sri Raghunatha Cakravarti, he also divided the verses into two sections. To the eighteen names of Vishnu Avatara Buddha he writes the words "astadash buddha", which clearly refers only to the Vishnu avatara. Next, on his commentary for the seven aliases of Shakya Simha he writes: "ete sapta shakya bangshabatirneh buddha muni bishete", meaning "the next seven names starting from Shakya-munistu are aliases of Buddha-muni [the human] who was born into the Shakya dynasty."
3. Gautama Buddha Was an Avesha Avatar of Vishnu
The first view held that Vishnu incarnated as Gautama Buddha, and the second view held that Vishnu Buddha was different from the atheistic Buddha. However, in many of his videos on DharmaNation, Sri Dharma Pravartaka Acharya has said that Gautama Buddha was an avesha avatar, which means he was not Vishnu Himself but a being empowered to fulfill his mission. In this video in particular, something that I found quite interesting was that Gautama Buddha never actually taught shunyavada (the doctrine of voidness). He also never claimed to be founding a new religion called Buddhism, and he was in fact following the Vedic scriptures. However, he only taught the negative half of the philosophy: the Buddha said that we are not the body, but he did not teach that we are the soul. There is no evidence in Buddhist scriptures that Gautama Buddha spread shunyavada; this misconception came about when Buddhism became systematized by Nagarjuna hundreds of years later. The Buddha's mission was to transition society away from animal sacrifice toward self-realization and meditation.
Vishnu Purana's aforementioned account of the Buddha can also be considered proof that he was an avesha avatar.
At the end of the day, there are several different viewpoints about the Buddha in Hindu scriptures; after all, this is quite a tricky subject. Not everyone believes in the same thing. I answered a lot more than just what you were asking, but the answer to your main question can be found under the first heading.
Bhagavata and all other Puranas which mention Buddha were Post Buddhist.
The Lord once more came to you as Buddha and taught you how to feel, how to sympathise with the poor, the miserable, the sinner, but you heard Him not. Your priests invented the horrible story that the Lord was here for deluding demons with false doctrines! True indeed, but we are the demons, not those that believed. And just as the Jews denied the Lord Jesus and are since that day wandering over the world as homeless beggars, tyrannised over by everybody, so you are bond-slaves to any nation that thinks it worth while to rule over you. Ah, tyrants! you do not know that the obverse is tyranny, and the reverse slavery. The slave and the tyrant are synonymous.(Complete Works of Swami Vivekananda Volume 5)
Ath the time when daityas started to follow Vedas, it became impossible for devas to destroy them. Devatas went to Lord Vishnu for help.
"Have compassion upon us, O lord, and protect us, who have come to thee for succour from the Daityas. They have seized upon the three worlds, and appropriated the offerings which are our portion, taking care not to transgress the precepts of the Veda. Although we, as well as they, are parts of thee, of whom all beings consist, yet we behold the world impressed by the ignorance of unity, with the belief of its separate existence. Engaged in the duties of their respective orders, and following the paths prescribed by holy writ, practising also religious penance, it is impossible for us to destroy them. Do thou, whose wisdom is immeasurable, instruct us in some device by which we may be able to exterminate the enemies of the gods."
Vishnu Purana 3:17
Lord Vishnu incarnated as Buddha to delude them from vedic teachings. So he established a new religion Buddhism.
Vishnu Purana 3:17. When the mighty Vishńu heard their request, he emitted from his body an illusory form, which he gave to the gods, and thus spake This deceptive vision shall wholly beguile the Daityas, so that, being led astray from the path of the Vedas, they may be put to death; for all gods, demons, or others, who shall be opposed to the authority of the Veda, shall perish by my might, whilst exercised for the preservation of the world. Go then, and fear not: let this delusive vision precede you; it shall this day be of great service unto you, oh gods!"
Vishnu Purana 3:18. In this manner, exclaiming to them, "Know!" (Budhyadwam), and they replying, "It is known" (Budhyate), these Daityas were induced by the arch deceiver to deviate from their religious duties (and become Bauddhas), by his repeated arguments and variously urged persuasions 5, When they had abandoned their own faith, they persuaded others to do the same, and the heresy spread, and many deserted the practices enjoined by the Vedas and the laws.
Later on gods defeated them.
Vishnu Purana 3:18.When the Daityas had thus declined from the path of the holy writings, the deities took courage, and gathered together for battle. Hostilities accordingly were renewed, but the demons were now defeated and slain by the gods, who had adhered to the righteous path.
This story is also mentioned in other scriptures.
Padma Purana 6:252 108-115. O you desiring the fruits of all times, what is the use of speaking much in this regard? A wise man should only utter the formula: ‘Salutation to Kṛṣṇa’. ‘Repeated salutations to Kṛṣṇa, Vāsudeva, Hari, the highest soul. I, bowing down repeatedly salute Govinda for the destruction of my affliction.’ O goddess, a man should, everyday, with devotion, mutter this formula. Being free from all sins, he would obtain Viṣṇu’s world. This lord Janārdana assumes different form(s) for the protection of all gods and of the worlds. I, with a desire to kill Tripura, worshipped Viṣṇu. The glorious one, taking up the form of Buddha, deluded his enemies. Those who were deluded by that teaching gave up all religious practices. (These) enemies of gods were killed by me with Viṣṇu’s missile. Viṣṇu, descending in a brāhmaṇa’s house at the end of Kali, will kill all the cruel Mlecchas. I have described along with all those appearances, all the forms of the lord of the world. O you of an auspicious face, what else do you want to listen to? I shall narrate it.
Srimad Bhagavatam 2:7:37 Asuras, the enemies of the celestials, became proficient in the knowledge of the Vedas, and having got made by Mayadanaba (an Asura of that name) and invisible city of velocity, engaged themselves in the destruction of the people. Thereupon the glorious God incarnated Himself as Buddha, and with a view to bring about a confusion of their understanding and to create avarice in them taught them many false religions in the guise of a Pashanda (one setting the true religious texts at naught).