Gemara

73b (Mishnah - R. Eliezer): If a widow has waited 30 days (for Yibum), since her Yavam must feed her, he can annul her vows;

Chachamim say that he cannot annul until she enters his Reshus (Yibum).

74a (Mishnah - R. Eliezer): The Yavam can annul vows of a Shomeres Yavam, whether there is one Yavam or two;

R. Yehoshua says, if there is one Yavam, he can annul her vows. If there are two, neither can annul them;

R. Akiva says, whether there is one Yavam or two, he cannot annul her vows.

R. Eliezer: If a husband can annul the vows of a woman whom he chose to marry, all the more so he can annul vows of a woman whom Shamayim acquired to him (i.e. through the death of his brother)!

R. Akiva: No. A husband can annul his wife's vows because she is special to him and to no one else. You cannot learn to a Yevamah, for others (other Yevamim) have a connection to her!

R. Yehoshua: Akiva, your objection applies only when there are two Yevamim, not when there is only one!

R. Akiva: A Yevamah is not acquired to her Yavam as fully as a wife is to her husband.

Question: Granted, R. Akiva holds that Ein Zikah (a Shomeres Yavam is not considered to be partially married to the Yavam), and R. Yehoshua holds that Yesh Zikah. What is R. Eliezer's reason?

Even if he holds that Yesh Zikah, there is not Breirah (retroactive clarification. I.e., we cannot say that now she is already considered partially married to the Yavam who will later do Yibum!)

Answer (R. Ami): The case is, Reuven (a Yavam) gave to her a Ma'amar (Kidushin of a Yevamah). R. Eliezer holds like Beis Shamai, who say that a Ma'amar fully acquires her. R. Yehoshua holds that this applies only when there is one Yavam;

It is unreasonable that Shimon (Reuven's brother) could have Bi'ah with her or give a Get to her, which would forbid her to Reuven, and Reuven can annul her vows!

R. Akiva holds that Ein Zikah.

Question: R. Elazar (ben Pedas, an Amora) holds that Beis Shamai say that a Ma'amar does not fully acquire a Yevamah. It merely eliminates Zikah to the other brothers. How can he explain R. Eliezer?

Answer: The case is, Beis Din ruled that the Yavam must feed her. R. Eliezer holds like Rav Pinchas:

(Rav Pinchas): Every woman intends that her vow be valid only if her husband will consent to it.

Suggestion: We can resolve Rabah's question (according to Beis Shamai, does a Ma'amar make Kidushin or Nisu'in)?

If it makes only Kidushin, he would not be able to annul (alone)!

(Mishnah): The vows of a Na'arah Me'orasah can be annulled by her father and Arus (together).

Rejection (Rav Nachman b'Rebbi Yitzchak): We can say that it makes Kidushin. R. Eliezer said that he can annul, i.e. together with the father.

Support (for R. Ami - Beraisa - R. Eliezer): Whether there is one Yavam or two, the Yavam can annul vows of the Shomeres Yavam;

R. Yehoshua says, he can annul if there is one Yavam, but not if there are two;

R. Akiva says, in either case he cannot annul.

R. Eliezer: A wife was not acquired at all to her husband before Kidushin. After marriage, the Kinyan is completed (and he can annul her vows). A Yevamah was partially acquired to her Yavam before he acquired her. All the more so, when she enters his Reshus, the Kinyan is completed!

R. Akiva: You cannot learn from a husband, for just like he had no attachment to her (before Kidushin), no one else did, either. A Yevamah was partially acquired to her Yavam by Shamayim. She was also partially acquired to other (Yevamim)!

R. Eliezer: R. Akiva, your objection applies only when there are two Yevamim, not when there is only one!

R. Akiva: We do not distinguish (regarding other laws) whether there is one Yavam or two, or whether or not he gave a Ma'amar. The same applies to Hafarah!

Ben Azai: It is a shame that I did not serve a Chacham like R. Akiva!

Rishonim

The Rif brings the Mishnah.

Ritva (DH Shomeres): R. Eliezer says that the Yavam can annul even when there are two Yevamim, i.e. each (Yavam) annuls. (This was the Hava Amina.) The Gemara establishes the Mishnah to be when one gave a Ma'amar or was obligated to feed her. R. Akiva says that even one Yavam cannot annul, even if he was obligated to feed her.

Nimukei Yosef (DH Shomeres): R. Yehoshua holds that Hafarah (of one Yavam, even if he gave a Ma'amar or was obligated to feed her) does not help when there are two Yevamim, i.e. unless each Yavam annuls.

Rambam (Hilchos Nedarim 11:23): A Yavam cannot annul the vows of a Shomeres Yavam before Yibum, even if he gave a Ma'amar to her, even if he is the only Yavam, and she is the only Yevamah.

Rambam (24): If a Na'arah Me'orasah fell to Yibum and her father was alive, the Yavam does not annul with the father. Rather, the father annuls alone. This is even if the Yavam gave a Ma'amar, for a Ma'amar does not acquire totally.

Rosh (10:6): The Halachah follows R. Akiva, for Ben Azai holds like him, and also Chachamim in our Mishnah (73b).

Ran (75a DH k'Shem): The Halachah follows R. Akiva, for Ben Azai holds like him, and R. Akiva refuted R. Eliezer and R. Yehoshua and they did not respond. I am unsure whether or not a father can annul when there is a Yavam. It seems that if she vowed before her Arus died, the father gets authority to annul, for the Yavam gets no authority to annul, so it is as if there is no Yavam. Tosfos explained that Beis Shamai say that the last Arus cannot annul it what the first Arus heard, therefore it is as if there is no Arus, and the father can annul alone. I do not know whether or not he can annul what she vowed after falling to Yibum.

Kesef Mishneh (Nedarim 11:24): Why didn't the Ran mention that the Rambam says that he can annul?

Yam Shel Shlomo (Yevamos 3:7): If the Yavam is not considered like an Arus regarding what she vowed in the life of her Arus, all the more so he is not like an Arus regarding what she vowed after her husband died, and the father can annul alone!

Question (Shirei Korban Yerushalmi Nedarim 33b DH Al Da'as): The Yerushalmi asked the Ran's question and did not settle it. Why didn't he mention this? Why does the Rambam rule that the father can annul? Perhaps he discusses when no Ma'amar was given. However, the Tur says so even when there was a Ma'amar! How did the Ran learn from Beis Shamai about a vow before falling to Yibum? We hold that the last Arus joins to annul!

Answer #1 (Keren Orah 75a DH Ran): The Yerushalmi had a Safek because it holds that Yesh Zikah. The Bavli holds that Ein Zikah, therefore it had no Safek.

Answer #2 (Shalmei Nedarim 75a DH b'Ran): R. Akiva Eiger asked why the Ran was in doubt. The Ran wrote (74a DH R. Akiva and 74b DH Oh Nisu'in) that R. Akiva holds that a Yavam is weaker than an Arus, and he cannot even annul in partnership! It seems to me that the Yavam is like a stranger, so his Hafarah does not join the father's, but perhaps the father can annul by himself! The Shirei Korban's text in the Yerushalmi said that it asked according to R. Akiva, who annuls, i.e. the father or the Yavam? What was the Safek? Surely the Yavam cannot annul! It seems that the Ran's text in the Yerushalmi was like the Pnei Moshe's. It asked, if a woman has many husbands (R. Yochanan holds that 100 men can partially Mekadesh her), according to R. Eliezer, who annuls?

Me'iri (74a DH Inyan): R. Eliezer holds that when there are two Yevamim, both of them annul her vows, even if one gave a Ma'amar and was obligated to feed her. Some say that in this case Chachamim agree that she wants her vows to be contingent on the Yavam's consent. However, we do not say so whenever a woman is fed. All agree that no one can annul vows of a Bogeres Arusah. R. Yehoshua holds that two Yevamim cannot annul, even together, because Ein Breirah. We require "Ishah Yeferenah", and it is not clear that he is her husband. Some say that the Halachah follows R. Yehoshua, for he is Machri'a (the intermediate opinion). Many say that the Halachah follows R. Eliezer.

Poskim

Shulchan Aruch (YD 234:7): A Yavam cannot annul the vows of a Shomeres Yavam before Yibum, even if he is the only Yavam, and even if he gave a Ma'amar to her (was Mekadesh her with money).

Beis Yosef (DH u'Mah she'Chosav v'Lo): The Tur says that a Yavam cannot annul alone. This is not precise. The Halachah follows R. Akiva, and the Tur himself says later that he cannot annul even with the father.

Taz (8): The Tur means that since the father's authority over her ceased from Nisu'in (with her first husband, who died), therefore, the Yavam cannot annul, even together with the father.

Note: This connotes that if she was widowed from Eirusin, the Yavam could annul with the father! I did not find anyone who says so.

Shulchan Aruch (11): If the Arus died while she is a Na'arah, she returns to her father's authority. He can annul in the day he hears even what she vowed and the Arus heard before he died. This is even if she has a Yavam and he gave a Ma'amar to her.