Nedarim 13 - Vow legalese

The Torah said, "One who vows a vow..." - why did it have to repeat the word? To teach us that if one connects his vow to some other prohibited thing, this thing must become prohibited through a declaration, and not be already prohibited. For example, he can say, "This is like a sacrifice to me," because an animal is not born a sacrifice, but rather someone designates it so. If he says, by contrast, "This is like a firstborn (animal) to me," then no vow is created, because a firstborn becomes consecrated as sacrifice at birth, by itself. Some make exception only for the firstborn, but the rule holds true otherwise.

Another important rule to remember: vows are valid only for material objects. For example, one cannot say, "I vow not to talk to you," but he can instead say "I vow for my mouth not to talk to you."

Art: Funeral of Firstborn by Nikolai Aleksandrovich Yaroshenko