2,752. Why Vows About Mitzvos Are Effective
Hilchos Nedarim 3:7
The reason vows about mitzvos take effect but oaths about mitzvos don’t is because one who takes an oath prohibits himself the object of his oath, while one who takes a vow renders the object of his vow prohibited to him. Accordingly, if someone makes an oath to annul a mitzvah, he is placing a prohibition upon himself when he’s previously bound by a conflicting oath from Sinai, i.e., to keep the mitzvos. His oath to prohibit can’t take effect while his oath to observe is already in effect. However, when someone renders something prohibited through a vow, the prohibition is about the thing itself, and the thing itself isn’t under an oath from Sinai.
Hilchos Nedarim 3:8
If you think about the words of the Torah, it seems that they concur with the explanation that our Sages received via our oral tradition. Regarding oaths of expression, Leviticus 5:4 says, “to do bad or to do good.” This refers to voluntary activities as was previously discussed, i.e., to eat, to drink, to fast, etc. Regarding vows, however, Numbers 30:3 says, “He shall do everything that comes out of his mouth,” without making any distinction between voluntary matters and mitzvos.