Nedarim 77: Nullification on Shabbat

Where yesterday's daf was a discussion about whwehtr or not a na'ara's vow can be nullified within 24 hours of hearing it or simply before nightfall, today's daf brings Shabbat into the picture.  Can a na'ara's vow be nullified by her father and/or her husband on Shabbat?  If the vow pertains to Shabbat (I won't eat although eating is commanded on Shabbat), nullification makes sense.  Ordinary vows can be nullified on Shabbat.  But what about a vow that does not pertain to Shabbat?  The rabbis discuss this and they elide that a woman's vow can be nullified on Shabbat for any reason.

If a halachic authority is called in, however, it can only be one authority rather than three people who would seem like judges.  That would be too much like work, which certainly is not permitted on Shabbat.  At the end of toddy's daf we find a note in the Steinsaltz explaining that on Shabbat, men must nullify the vow in their hearts and then speak some action aloud.  For example, if his wife has vowed not to eat on Shabbat, he can say "Take it and it eat,"  which demonstrates his internal nullification, action rather than simple intention, and the imposition of his nullification, where she is forced to eat.

The Gemara continues, questioning whether vows can be nullified by an authority who is sitting or who is standing.  It is noted that nullification requires that the person making the vow expresses his/her regret for having made the vow in the first place.  Further, as said by Rav Zevid, if you make no vow, there is no sin in you" (Deuteronomy 23:23).

There is a short conversation regarding how men should nullify their wives' vows.  They can repeat a statement that nullifies her vows three times, or they can have her take an action, on Shabbat, that is different from other days.  When a husband tries to nullify a vow and his efforts don't work the rabbis tell us that he has not said anything.   The rabbis end today's daf with the start of a conversation about whether or not a husband can nullify his wife's vows on his own or whether he requires a halachic authority.

Why not let a woman fast on Shabbat if she feels that this is what she needs to do for some reason?