The Mishna (20b) teaches that if a man says, “Hareini nazir,” and his wife agrees, saying, “Va-ani – and I,” the husband can choose to be mefer her acceptance of the status of nezirut, even though his own commitment remains intact. If, however, a woman says hareini nezira – “I accept upon myself the rules of a nazir” – and her husband says va-ani in response, the husband can no longer annul his wife’s vow of nezirut, and they will both become nezirim.
At first, the Gemara tries to use this ruling to conclude that once such a neder has been approved by the husband, he cannot change his mind because he would essentially be annulling his own neder. That is to say, once he erases the neder made by his wife, it will become clear that his own statement of va-ani no longer refers to a meaningful commitment and his commitment to nezirut will disappear, as well. After some discussion, however, the Gemara explains 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 – so emphatic that it cannot be undone.
The Ran in Massekhet Nedarim (21b) explains that the statement that he made – va-ani – is seen as a statement of emphatic approval, as if he said “I approve of your neder forever.” By accepting personal nezirut 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 would have anticipated that he would have clearly said that he was accepting nezirut upon himself even as he was forbidding his wife from keeping her vow.