The gemara says that one cannot establish a veset during the days of being a Nida, but according to R. Yochanan and Reish Lakish she can establish a veset during days of zivah. However, Tosafos (d.h. Rav Huna) explains that according to Rav Huna Bar Chiya in the name of Shmuel a woman cannot even establish a veset during days that she can potentially become a zavah. Therefore, a woman can never establish a veset that is shorter than 18 days since the 11 days after the 7 days of Nida are days that she can potentially become a zavah. But, Tosafos says that we pasken like Rav Yochanan against Shmuel, therefore the only time she can't establish a veset is during the days of actual Nidah, but can establish a veset during days of actual zivah (and certainly during days of potential ziva). The only exception to when she can establish a veset even during the actual days of Nida is at the very end of the perek - if 2 of the 3 times she saw blood it was not during days of nidah or zivah, but the third time it was during the days of nidah (and certainly if it was in the days of zivah), she would establish a veset since we consider the initial bleeding on the third time to be "extra blood".
The Shulchan Aruch doesn't discuss the issue of establishing vestos during days of Nidah and Zivah. However, the Nekudas HaKesef (189 around si'if 19) writes that the poskim (rashba and tur) rule that a woman cannot establish a veset during days that she is fit to be a zavah, therefore a woman can never establish a veset for a haflaga that is 18 days or less. The Nekudas HaKesef explains that although the Shulchan Aruch omits this din because we are machmir not to differentiate between nidah and zivah, that is only to be machmir. The Nekudas HaKesef takes a very strong stance that if a woman would establish a veset for 17 days, she must be machmir for the veset, but cannot be lenient to avoid being concerned for the standard vestos that a woman who doesn't have an established veset would need to be concerned about. The Torah HaShlamim earlier in the siman (27) argues on the Nekudas HaKesef and writes that since vestos are d'rabonon we can be lenient to not be concerned at all for a veset that is shorter than 18 days. Since the entire minhag of not keeping track to differentiate between days of Nida and Ziva is because it is too confusing, even for vestos it is not necessary to keep track and one can always assume that a woman would establish a veset even during days of zivah, even if it results in leniency. The logic of the Toras HaShlamim is difficult. How can one adapt a minhag to treat every nida like a zava when it will result in a leniency for vestos because we will ignore the problem of establishing a veset during days of zivah? Perhaps we can combine the opinion of Tosafos that we pasken like Rav Yochanan against Shmuel and one can establish a veset during days that are fit to become a zava and it would be treated like a regular veset.