In the Canki Sutta, the Buddha hints that even the learned brahmans themselves are unsure whether the Vedas are true, by their own empirical validation:
"And among the brahman seers of the past, the creators of the hymns,
the composers of the hymns — those ancient hymns, sung, repeated, &
collected, which brahmans at present still sing, still chant,
repeating what was said, repeating what was spoken — i.e., Atthaka,
Vamaka, Vamadeva, Vessamitta, Yamataggi, Angirasa, Bharadvaja,
Vasettha, Kassapa & Bhagu: was there even one of these who said, 'This
we know; this we see; only this is true; anything else is worthless?'"
"No, Master Gotama."
"So then, Bharadvaja, it seems that there isn't among the brahmans
even one brahman who says, 'This I know; this I see; only this is
true; anything else is worthless.' And there hasn't been among the
brahmans even one teacher or teacher's teacher back through seven
generations who said, 'This I know; this I see; only this is true;
anything else is worthless.' And there hasn't been among the brahman
seers of the past, the creators of the hymns, the composers of the
hymns... even one who said, 'This we know; this we see; only this is
true; anything else is worthless.' Suppose there were a row of blind
men, each holding on to the one in front of him: the first one doesn't
see, the middle one doesn't see, the last one doesn't see. In the same
way, the statement of the brahmans turns out to be a row of blind men,
as it were: the first one doesn't see, the middle one doesn't see, the
last one doesn't see. So what do you think, Bharadvaja: this being the
case, doesn't the conviction of the brahmans turn out to be
The Buddha also rejected various Vedic practices as being superstitious.
In the Sigalovada Sutta, the Buddha rejected the ritualism behind the Vedic bath prayer ceremony practised by a young householder by the name of Sigala, and put a new spin on it, according to the Buddha Dhamma.
More can be found in the Brahmana Samyutta (SN 7):
In SN 7.8, the Buddha rejected food ritually chanted over from the
Vedic fire sacrifice because he discouraged superstition. By
accepting the food, he would be validating the notion that the
sacrificial milk rice is special and meant for noble ones.
In SN 7.7, the Buddha says that austerities does not make one pure
and chanting mantras does not make one a brahmin.
In SN 7.9, the Buddha challenged the superstition of caste by birth.
In SN 7.21, the Buddha enlightened a brahmin who held the
superstitious view that purification rites using water can wash away
In SN 42.6, the Buddha rejected the use of prayers to help the deceased go to heavan.
In DN 2, the Buddha forbade his monks from doing fire oblations or fire sacrifices, practising astrology, reading omens, calculating auspicious dates for marriages, consecrating sites for construction and worshipping the Sun.