Gemara

111b (Mishnah): If a woman vowed in the life of her husband not to benefit from her Yavam, we force the Yavam to do Chalitzah. If she vowed after falling to Yibum, we request of him to do Chalitzah;

If she intended to prevent Yibum, even if she vowed in her husband's lifetime we request of the Yavam to do Chalitzah.

112a (Mishnah): At first, they used to say that if a man's wife vowed not to have Bi'ah with any Yisrael (this shows that Bi'ah is painful for her, so) we force him to divorce her and pay a Kesuvah;

Later, Chachamim changed the law, lest a woman desire to marry someone else and vow in order to get divorced. Therefore, her husband annuls the part of the vow that forbids him. She is permitted to him and permanently forbidden to other men.

Question: If a married woman vowed not to have Bi'ah with any Yisrael (and fell to Yibum), what is the law?

Did it cross her mind that she might fall to Yibum (and she intended to forbid Yibum), or not?

Answer #1 (Rav): The Yavam is not like the husband (it did not cross her mind).

Answer #2 (Shmuel): The Yavam is like the husband.

Support (for Rav - Abaye - Mishnah): If a woman vowed in her husband's lifetime not to benefit from her Yavam, we force him to do Chalitzah.

If it crossed her mind, we should only request that he do Chalitzah!

Rejection: The case is, she had children. She did not expect that her children will die and then her husband.

Inference: If she had no children, we only request that he do Chalitzah.

Objection (Seifa): If she intended to prevent Yibum, we request that he do Chalitzah.

The Seifa should distinguish without changing her intent, i.e. we force him to do Chalitzah only if she had children. If not, we only request!

Conclusion: Rather, whether or not she had children, we force him to do Chalitzah, like Rav.

Rishonim

Rif and Rosh (11): When a married woman vowed not to have Bi'ah with any Yisrael, if we would say that it crossed her mind that she might fall to Yibum, we would not force the Yavam to do Chalitzah. The Halachah follows Rav; she is not like a Moredes. However, if she vowed after falling to Yibum she is a Moredes.

Nimukei Yosef (DH Netulah): If it crossed her mind that she might fall to Yibum, the enactment (lest one vow in order to get divorced) would apply. If not, the Mishnah Rishonah applies, so he must do Chalitzah.

Rambam (Hilchos Yibum 2:15): If a Yevamah had vowed in the life of her husband not to benefit from her Yavam, we force the Yavam to do Chalitzah and she collects her Kesuvah. If she vowed after he died, we request of him to do Chalitzah. If he does not want she is Moredes. Similarly, if she intended to prevent Yibum, even if she vowed in her husband's lifetime we do not force him to do Chalitzah unless she rebelled, and she leaves without a Kesuvah.

Ra'avad: Here we force or request Chalitzah immediately. This is unlike a Moredes, in which case we do not strive to permit her until after a while.

Magid Mishneh: The Rambam (2:10) says that whenever a Yevamah rebels she forfeits her Kesuvah and we force him to do Chalitzah. He explains our Gemara to discuss when she wants her Kesuvah. The Ra'avad agrees; he argues only about when we force. His opinion is difficult, for sometimes we force a husband to permit a Moredes immediately, e.g. when she says 'he is repulsive to me'. Vowing Hana'ah from the Yavam is like this. However, the primary opinion is that we never force the Yavam.

Rebuttal (Mishneh l'Melech): The Rambam says 'we do not force him to do Chalitzah unless she rebelled'. This implies that Stam, vowing is not Meridah! Moredes is when she wants to be married to him, but wants to pain him.

Poskim

Shulchan Aruch (EH 165:2): If a woman vowed not to benefit from her Yavam while her husband was alive, or she vowed from any Yisrael (and fell to Yibum), we force the Yavam to do Chalitzah. If she vowed after her husband died, we request the Yavam to do Chalitzah. Even if she vowed while her husband was alive but she intended to forbid Yibum, we request the Yavam to do Chalitzah.

Beis Yosef (DH Yevamah): Rashi says that a vow without intent to forbid Yibum is when there was enmity between her and the Yavam. The Magid Mishneh explains Rashi to say that that when there was enmity we suspect that she intended to forbid Yibum.

Rebuttal (Mishneh l'Melech): Rashi holds that when there was no enmity she intended to forbid Yibum.

Beis Yosef (DH Kosav): The Tashbetz (2:260) says that if she later permitted her vow she can do Yibum. We do not say that since Yibum was forbidden when she fell, she is always forbidden.

Beis Shmuel (4): The forcing is with sticks.

Beis Shmuel (5): Rashi, the Ba'al ha'Ma'or and the Rashba explain that if she vowed in her husband's lifetime, the vow does not include the Yavam. Since some permit her to the Yavam, we cannot force him to do Chalitzah, like we say (Sa'if 1) about one who does not intend for the Mitzvah.

Beis Shmuel (6): When we request the Yavam to do Chalitzah and he refuses, she is considered a Moredes, even though there is no solution to permit her to him. The Rema brings the Rashba's opinion that she is not a Moredes whenever there is no solution.

Question: If she cannot do Yibum, also her Chalitzah is Pasul!

Answer (Taz): Since there is no alternative, this is like b'Di'eved, and the Chalitzah is Kosher. Further, she is considered fitting for Yibum; the vow is a side Isur.