Halakhah for Nedarim 15:14

And so from this reason, an oath cannot rest upon an oath [about the same matter], but a vow can rest upon a vow [about the same matter]. As with an oath, behold once a man has himself entered into the partition of [something's] existence, as we have said, even if he repeats his word that he is entering there a thousand times, the entrance of his body into another place is done [only] one time. And this that he does after that is only repeating words pointlessly. But with a vow in which he is like one that accepts upon himself that something permissible is as if it is forbidden; with each time that he repeats his acceptance, he adds [a further] prohibition if he [breaks his vow]. And hence he is liable for each and every one. And it is the same matter itself that an oath cannot rest upon the matter of a commandment (Nedarim 16a), but a vow can rest even upon the matter of a commandment. As one who makes an oath speaks about his body, and his body is already obligated in that matter [of the commandment] from Mount Sinai. But with a vow, he is only speaking about the object that he wants to bring into the category of the forbidden, and about this specific object, he was never obligated. And hence [his new] prohibition rests upon it. And we do not feed a person something that is forbidden for him [because of an oath or a vow]. And this is what they, may their memory be blessed, said (Ran on Nedarim 8a, s. v. Vehalo mushva) that the one who makes an oath forbids himself to the object and the one who makes a vow forbids the object to himself.