A few times in the gemara it raises the issue whether vestos are considered d'oraysa or d'rabonon. There are many aspects to this issue, most fundamentally to define the concept of vestos d'roaysa. In the next post I hope to discuss the more fundamental aspect. However, today I want to point out that within the opinion of vestos d'oraysa, there seems to be a machlokes between Rashi and Tosafos. Rashi (d.h. d'oraysa) writes something cryptic - vestos are d'oraysa from a halacha l'moshe mi'sinai, that we establish her to have safeik tu'mah. Rashi seems to understand that the halacha l'moshe mi'sinai itself is a halacha that only establishes a questionable status of tu'mah, but doesn't establish her as definitely tamei. Tosafos (d.h. afilu) raises a question within the opinion that vestos are d'rabonon, how can a woman be permitted m'dorasya at the time of her veset, the gemara in shavuos seems to learn out from a pasuk that she is forbidden. Tosafos answers that she is certainly forbidden at the time of her veset, but after she waits enough time to go to the mikvah after the veset passes, and the husband doesn't know if she went to the mikva, according to the opinion that vestos are d'rabonon she is permitted without him verifying her status. However, according to the opinion that vestos are d'oraysa, Tosafos writes that we assume she "definitely" saw, therefore a chance of immersion doesn't take away the "definite" status. Tosafos seems to argue on rashi and understands that the assumption she saw is a va'dai, not a safeik.
The gemara on 15b says that when a woman has a veset, her husband can calculate the days of her veset and assume she went to the mikvah when enough time has passed. The gemara refers to the reason that he can be so lenient is because it is only a safeik whether she saw. According to rashi this fits even with the opinion who says that vestos are d'oraysa but according to tosafos it only fits with the opinion that vestos are d'rabonon.