Mishnah 6 in Maseches Yevamos discusses the status of a shomeres yavam with regard to annulling vows: is the yavam considered a husband for this purpose, so that he may annul her vows?
"Shomeres yavam" - a woman whose husband died and upon whom neither yibum nor chalitzah has yet been performed by her husband's brother. The Mishnah discusses her in both situations: whether only one brother remains who is able to perform yibum or chalitzah, or whether two brothers remain.
Three opinions are stated in the Mishnah:
Rabbi Eliezer says: "Yafer" - the yavam may annul her vows. The Gemara clarifies: when there are two brothers, which of them annuls? It explains that the case is one in which one of the brothers performed maamar with her, which is a rabbinic form of betrothal. Rabbi Eliezer holds that maamar effects a complete and absolute acquisition, and she is like an actual betrothed woman to that yavam. Therefore, if she was widowed from betrothal and her father is alive, the yavam who performed maamar with her annuls her vows in partnership with her father.
Rabbi Yehoshua: the annulment is effective only when there is a single yavam; where there are two yevamin, the yavam does not annul. And even if one of them performed maamar with her, Rabbi Yehoshua holds that the maamar is not effective as a full betrothal but only as a rabbinic betrothal, so there is no actual betrothal here, and therefore the annulment would not be effective.
Rabbi Akiva: the yavam does not annul at all - neither where there are two yevamin, nor even where there is a single yavam.
Rabbi Eliezer's kal vachomer:
Rabbi Eliezer brings a proof for his position, showing why the yavam should be regarded as one who is able to annul vows: "Mah im ishah shekanah hu le'atzmo" - a woman whom he acquired himself through betrothal, he annuls her vows together with her father while she is betrothed. A woman who was given to him by Heaven, by the power of the Torah and the Holy One, blessed is He - is it not right that he should annul her vows? A kal vachomer that he should have the power to annul.
Rabbi Akiva's response:
Rabbi Akiva said to him: "Lo" - this is not a valid argument. "Im amarta be'ishah shekanah hu le'atzmo, she'ein la'acherim bah reshus" - there the annulment is effective because that acquisition has a unique quality: a woman he betrothed has no connection whatsoever to any other man, and she is his wife exclusively. "Tomar be'ishah shehiknu'ha lo min hashamayim, sheyeish la'acherim bah reshus" - regarding a yevamah, to whom others also have a connection, for when she falls to yibum she may have more than one yavam. It emerges that the bond is not so strong: it does indeed come from Heaven, but at the same time it is not exclusive to him.
Rabbi Yehoshua's objection:
Rabbi Yehoshua said to him regarding Rabbi Akiva's argument: "Akiva, devarecha bishnei yevamin" - your argument is well taken where there are two brothers, since then the connection is not exclusive. "Mah tomar beyavam echad" - but in a case where there is only one yavam, the exclusivity does exist, and his acquisition comes from Heaven, so it stands to reason that his bond should be stronger.
Rabbi Akiva answered him: "Ein hayevamah gemurah layavam keshem shehaarusah gemurah la'ish" - even with a single yavam, the yevamah's connection to him is not as strong and absolute as a betrothed woman's connection to her husband. By its very definition, the bond between a yavam and a yevamah is not equal in force to the bond of betrothal between a man and his wife.
In summary: in this Mishnah the Tannaim disagree as to whether the yavam annuls the vows of a shomeres yavam. Rabbi Eliezer permits it both with one yavam and with two, on the strength of maamar, which in his view effects a complete acquisition, and on the strength of a kal vachomer from a woman he acquired himself. Rabbi Yehoshua permits it only with a single yavam, since maamar is not a full betrothal. Rabbi Akiva disqualifies it in every case, because others have a connection to the yevamah, and because the yevamah's connection to the yavam is not as complete as a betrothed woman's connection to her husband.