The gemara questions again whether vestos are d'oraysa or d'rabonon. It is clear from the sugya that the difference between the two approaches is when a woman forgot to do a bedika during the time of her veset, but does a bedika after the time of her veset and comes out clean, whether she can assume a status of taharah. The opinion who holds vestos are d'oraysa require he to assume that she bled during her veset and because she failed to do a bedika she didn't notice the blood.
One of the major questions regarding this machlokes is raised by the maharatz chiyus 15a. In all other areas of halacha we apply a concept of chazaka. When something occurs 3 times, we assume it will happen again. What is the rationale to say that vestos are only d'rabonon - shouldn't it be d'oraysa due to the chazaka?
There are many different approaches to this. The Pischei Teshuva (184:3) cites the Nodeh B'yehuda and Chasam Sofer who say that the chazaka of being tehora contradicts the chazaka of orach b'zmano ba, therefore we don't have a d'oraysa chazaka telling us that she became a nidah since it is countered by the status quo of not being a Nidah. They continue to explain that this works only when looking back trying to determine whether she became a Nidah during the veset. However, when one looks forward to be concerned that a woman will become a Nidah, the status quo doesn't help to say she will always remain in the state of tahara since we know for certainty that she will NOT always remain in this state. They apply the concept of שמא מת לא חיישינן, שמא ימות חיישינן - we aren't concerned that a person died because they have a chazaka of being alive, yet we are concerned that they will die. Therefore, even if vestos are only d'rabonon, that means that after the time of the expected period, she is still assumed to be tahora, but during the time that she is expecting her period she is forbidden according to the Torah from having relations since there is a d'oraysa fear that she will become a Nidah during the veset time.
An alternate approach is indicated by the Chavos Da'as (cited in Pischei Teshuva 18). Although a woman has a chazaka that she will become a Nida during her veset, the lack of hargasha proves that she didn't become a Nidah. Therefore, although the chazaka may be d'oraysa the fact that she didn't have a hargasha creates a rei'usa in the chazaka. Based on this the Chavos Da'as holds that if a woman did something during her veset to mask her hargasha, such as doing a bedika and losing the cloth, or even urinating, then the lack of hargasha cannot prove that she is tahorah. She would need to be concerned that she actually had a hargasha without realizing it and is tamei. It would also seem based on this approach that the chavos da'as would agree with the nodeh b'yehuda and chasam sofer - she would have to be concerned during the time of her veset that she may get a hargasha and start bleeding. This is a d'oraysa concern even according to the opinion who holds that vestos are only d'rabonon.
Both these approaches seem to assume that the chazaka of the veset is itself a regular chazka, just that there is something counter acting the chazaka demoting the chazaka to be only a d'rabonon concern and not d'oraysa.
The Chazon Ish has a different approach. He understands that all agree that chazaka wouldn't apply here since there are so many variables that can impact a woman's menstrual cycle. Even the opinion who holds that vestos are d'oraysa doesn't attribute it to chazaka but rather to a special halacha l'moshe misinai, as rashi writes on both 9a and 15a. The opinion who holds vestos d'rabonon holds that there is no halacha l'moshe misinai. Following this approach, it is very understandable why rashi holds that according to the opinion that vestos are only d'rabonon, even without a bedika to confirm she is tahora, we can assume she is tahora. This is apparent in rashi in his comment at the start of the sugy, and against Tosafos who seems to say that she is only tahora after confirming it through a bedika at some point later on. Based on this approach, we aren't concerned about doing an activity that may have masked the hargasha because the entire concern of having a hargasha and seeing was created by the rabbonon, not by a real chazaka.