What are Buddhist arguments against Vedic validity because of its authorlessness?

A large section of the Tattvasangraha is devoted to arguments for and arguments against the reliability of the Vedas. The Tattvasangraha by the 8th century Buddhist Shantarakshita is an eight-century works detailling on the various philosophical systems from around that time (including both Buddhist and non-Buddhist).

From Wikipedia: “Śāntarakṣita defended a synthetic philosophy which combined Madhyamaka, Yogācāra and the logico-epistemology of Dharmakirti into a novel Madhyamaka philosophical system”.

The first couple of verses of the arguments against goes as follows:

All this is the product of the false pride of the twice-born people. There is no truth in this, even by the slightest change.—(2352)

Even though there is no author, the Veda cannot be regarded as saying what is true;—because it is devoid of those excellences in its source which would make it truthful;—just as in the absence of defects, the word is not regarded as false,—(2353)

It is only in the case of the works of persons that the question arises as to whether the excellences are there or not. hence there is no need for such an enquiry in the case in question; and we have not the slightest idea of there being any excellence at all.—(2354)

Thus, inasmuch as the causes of truthfulness and falsity,—in the share (a) of wisdom and mercy and (b) of absence of mercy, etc.,—are not there, the said two qualities cannot belong to the Veda.—(2355)

Thus the Veda is reduced to futility,—like such sentences as ‘six cakes’.—If it be argued that “meanings are actually comprehended (from Vedic sentences), in respect of actions and active agents”,—[the answer is that] there may be such comprehension in cases where there are explanations supplied by men,—as in the case of the doings of Urvaśī,—even though the words (of the Veda) by themselves do not really convey any such meaning at all,—as held by you.—(2356-2357)

This excerpt from DN 13 may answer your question sufficiently.

“Well, of the brahmins who are proficient in the three Vedas, Vāseṭṭha, is there even a single one who has seen Brahmā with their own eyes?”

“No, Master Gotama.”

“Well, has even a single one of their teachers seen Brahmā with their own eyes?”

“No, Master Gotama.”

“Well, has even a single one of their teachers’ teachers seen Brahmā with their own eyes?”

“No, Master Gotama.”

“Well, has anyone back to the seventh generation of teachers seen Brahmā with their own eyes?”

“No, Master Gotama.”

“Well, what of the ancient hermits of the brahmins, namely Aṭṭhaka, Vāmaka, Vāmadeva, Vessāmitta, Yamadaggi, Aṅgīrasa, Bhāradvāja, Vāseṭṭha, Kassapa, and Bhagu? They were the authors and propagators of the hymns. Their hymnal was sung and propagated and compiled in ancient times; and these days, brahmins continue to sing and chant it, chanting what was chanted and teaching what was taught. Did they say: ‘We know and see where Brahmā is or what way he lies’?”

“No, Master Gotama.”

“So it seems that none of the brahmins have seen Brahmā with their own eyes, and not even the ancient hermits claimed to know where he is. Yet the brahmins proficient in the three Vedas say: ‘We teach the path to the company of that which we neither know nor see. This is the only straight path, the direct route that leads someone who practices it to the company of Brahmā.’

What do you think, Vāseṭṭha? This being so, doesn’t their statement turn out to have no demonstrable basis?”

“Clearly that’s the case, Master Gotama.”

“Good, Vāseṭṭha. For it is impossible that they should teach the path to that which they neither know nor see.

Suppose there was a queue of blind men, each holding the one in front: the first one does not see, the middle one does not see, and the last one does not see. In the same way, it seems to me that the brahmins’ statement turns out to be comparable to a queue of blind men: the first one does not see, the middle one does not see, and the last one does not see. Their statement turns out to be a joke—mere words, void and hollow.

What do you think, Vāseṭṭha? Do the brahmins proficient in the three Vedas see the sun and moon just as other folk do? And do they pray to them and beseech them, following their course from where they rise to where they set with joined palms held in worship?”

“Yes, Master Gotama.”

“What do you think, Vāseṭṭha? Though this is so, are the brahmins proficient in the three Vedas able to teach the path to the company of the sun and moon, saying: ‘This is the only straight path, the direct route that leads someone who practices it to the company of the sun and moon’?”

“No, Master Gotama.”

“So it seems that even though the brahmins proficient in the three Vedas see the sun and moon, they are not able to teach the path to the company of the sun and moon.

But it seems that even though they have not seen Brahmā with their own eyes, they still claim to teach the path to the company of that which they neither know nor see.

What do you think, Vāseṭṭha? This being so, doesn’t their statement turn out to have no demonstrable basis?”

“Clearly that’s the case, Master Gotama.”

“Good, Vāseṭṭha. For it is impossible that they should teach the path to that which they neither know nor see.