Why a Vow Can Bind Against a Commandment but an Oath Cannot

In what way is a vow more stringent than an oath? If one says "konam be the sukkah that I make, the lulav that I take, the tefillin that I lay," by a vow he is forbidden, but by an oath he is permitted, for one does not swear to nullify a commandment, as it is said "he shall not profane his word" (Numbers 30:3) - his own word he shall not profane, but he may profane it concerning the things of Heaven. What is the difference? Of a vow it is written "if a man vows a vow... he shall not profane his word," but of an oath too it is written "or swears an oath... he shall not profane his word." Abaye said: in one case he says "the benefit of the sukkah is forbidden to me," and in the other he says "I swear that I will not benefit from the sukkah." Rava said: were the commandments given for benefit? Rather, in one case he says "sitting in the sukkah is forbidden upon me," and in the other "I swear that I will not sit in the sukkah." Rabbi Yehudah ben Beteira says: one who swore to fulfill a commandment and did not fulfill it is liable, and it is a matter of reasoning: if for an optional matter, for which he is not bound from Sinai, he is liable, then for a commandment, for which he is bound from Sinai, is it not right that he be liable? They said to him: no - if you say so of an oath about an optional matter, where a negative oath is like an affirmative one, will you say so of an oath about a commandment, where a negative is not like an affirmative? One might think one who swore to nullify a commandment and did not is liable; Scripture says "to do evil or to do good" - just as doing good is optional, so doing evil is optional, excluding one who swore to nullify a commandment, for that is not within his power. Why must a master force his ward to a Nazirite vow, but need not force him to other vows or to an oath? Because Scripture says "to do evil or to do good" - excluding harming others, which is not within his power.