The gemara learns out from a pasuk והזרתם את בני ישראל מטומאתם, that there is an obligation for one to separate from his wife around the time of the veset. There is a discussion in the gemara whether the concept of vestos are d'oraysa or d'rabonon. Tosafos in Yevamos 62b seems to assume that whether the issur in our gemara to separate at the time of the veset is d'oraysa, is dependent on the machlokes whether vestos are d'oraysa or d'rabonon.
Since we are speaking about a case where the woman has a veset ka'vuah - an established veset of 3 times, it seems that it should be d'oraysa. Why would it be different than any other chazaka? What is the peshat in the opinion who says that vestos are d'rabonon?
The Chazon Ish (Y.D. 80) writes that there are two ways to explain why veset would be different from a normal chazaka and only be d'rabonon. One approach is that when the veset comes and she doesn't get her period, the absence of a hargasha - feeling that her period is here, indicates that she didn't get her period. Therefore, a woman who is not able to use the absence of a hargasha as a proof such as a shoteh - fool, would have to be concerned m'doraysa just like any other chazaka. The second approach is that this type of chazaka is fundamentally different than a standard chazaka because there are many factors that contribute to when a woman will see and they are not all repeated each month. Therefore, the very nature of this type of chazaka, even without realizing that a hargasha was missed, would downgrade the concern to only being d'rabonon.
The difference between the two approaches would be the sevara of the Nodeh B'yehuda. The N.B. asks that the entire discussion of wheter vestots d'rabonon would seem to make sense after the veset passes and she doesn't have a hargash or period. But, when she is standing before the time that she is expecting her period, the prohibition to be with her husband is d'oraysa in nature because she has a chazaka that her period is on its way. Therefore, the N.B. says that even according to the opinion that vestos are d'rabonon (as the shach assumes l'halacha), the requirement to separate prior to the arrival of the veset would be mandated by the Torah. Just as we find that we aren't concerned that someone had died because it goes against chazaka, but are concerned that he will die in the next minute since it doesn't go against chazaka - here too, we aren't concerned that the veset came, but are concerned that it will come. Based on this the N.B. argues that even if vestos are only m'drabonon, the issur mentioned in our sugya to separate from one's wife prior to the veset is d'oraysa.
However, the Chazon Ish points out that the argument of the N.B. is assuming that the chazaka of veset is similar to all chazakos elsewhere. But, if we take the second approach that the very nature of this chazaka is weaker than standard chazakas, then even the requirement to separate prior to the arrival of the veset would not be mandated by the Torah. This approach of the Chazon Ish is against the Nodeh B'yehuda, Chasam Sofer and Chavos Da'as (184:10) who all assume that it is the lack of hargasha which downgrades the concern of the veset to only be d'rabonon.
Even according to the sevara of the N.B. that separating prior to the arrival of the veset is d'oraysa, because we consider it to be a real chazaka; Rav Moshe (igros moshe y.d. 4:14:184:2) explains that it would only apply to a woman who gets her period at any time during the o'nah, but the exact time isn't predictable. However, when a woman always gets her period in the evening or always in the morning, the chazaka would only require prisha at that time. The issur that is introduced by our gemara could still only be d'rabonon. The sevara of the N.B. wouldn't apply when the period always comes in the evening, therefore the issur all day long would be out of concern that bi'ah may induce the period to come earlier than usual, which may only be d'rabonon.