“As all that he gives out from his mouth (utters) he shall do,” speaks of vows involving prohibitions. These prohibitions (i.e. “I won’t drink wine...”) are created only by the word of man. Thus, there only applies one prohibition, “As all that he gives out from his mouth (utters) he shall do.”
This is the reason behind Maimonides’ ruling (-ibid, 13:25), “Our Sages stated (-Nedarim 22a), ‘Anyone who takes a vow is considered as having built a private altar (forbidden since the time that the Holy Temple was built in Jerusalem).’ And if he transgressed and took a vow, it is a mitzvah to ask (a sage) to absolve it, so that he will not have an obstacle before him. When does the above apply? With regard to vows involving prohibitions. However, with (regard to) vows involving the consecration of articles, it is a mitzvah to fulfill them and not to ask for them (absolution) only out of being (financially) pressed, as it is stated (-Psalms 116:14), ‘I will fulfill my vows to G-d.’”
