Choosing to become a nazir is essentially a type of neder. Thus, a woman who accepts upon herself to be a nezira can have her statement annulled by her father or husband if they object to it on the day that they hear of it, just like any other case of neder. The first Mishna in the fourth perek rules that in a case where a woman says hareini nezira – “I am hereby a nazir” – and her husband says in response va-ani – “I am, as well” – the husband can no longer nullify his wife’s vow of nezirut.
The Gemara in Massekhet Nedarim (70a) initially tries to use this ruling to conclude that once the woman’s neder has been approved by her husband, he cannot change his mind and do hafara on the neder (nullify the vow) later that day, since a hakama (a statement of support for the vow) that has been made cannot be changed. The Gemara explains, however, that there is a different rule that is applicable in this case. In fact, we do not consider the husband’s statement as merely accepting nezirut himself; rather we understand it to be an emphatic approval of his wife’s statement.
The Ran in Massekhet Nedarim explains that the statement that he made – va-ani – is seen as a statement of approval, as if he said “I approve of your neder forever.” In this case, it appears that his statement expresses his approval. By accepting nezirut himself we understand him to be saying that he is so comfortable with the idea of nezirut that he is willing to accept it upon himself. Had he, in fact, desired to reject his wife’s vow, we anticipate that he would have clearly said that he was accepting nezirut upon himself even as he was forbidding his wife from keeping her vow.