(24) You must fulfill what has crossed your lips and perform what you have voluntarily vowed to your God יי, having made the promise with your own mouth.

§ The Sages taught in a baraita: The verse states: “That which is gone out of your lips you shall keep and do; as you have vowed as a gift to the Lord your God, which you have promised with your mouth” (Deuteronomy 23:24). “That which is gone out of your lips”; this is a positive mitzva. “You shall keep”; this is a prohibition, as the phrase “you shall keep” is a warning to keep oneself from sinning. “And do”; this is an admonition to the court to make you fulfill your vow. “As you have vowed”; this is referring to a vow-offering. “To the Lord your God”; this is referring to sin-offerings, guilt-offerings, burnt-offerings, and peace-offerings, teaching that one must keep his word and bring them. “As a gift”; this is understood in its literal sense to be referring to a gift-offering. “Which you have promised”; this is referring to objects consecrated for Temple maintenance. “With your mouth”; this is referring to vows of charity, to which one commits himself with his mouth.

That is that we were commanded to fulfill everything that we we take upon ourselves in speech, whether in an oath, a vow, charity, a sacrifice or anything else besides them. And that is His, may He be exalted, saying, "What issues from your lips shall you observe" (Deuteronomy 23:24). And even though they have already separated the words of this verse and arranged each word of it for various content, the intention that nevertheless comes out of all that [they] said to you is that it is a positive commandment for a person to fulfill everything that he accepts upon himself, whatever it may be. And one who violates it, [also] violates a negative commandment; and behold I will explain this in my mention of the negative commandments (Sefer HaMitzvot, Negative Commandments 157). And in the Sifrei (Sifrei Devarim 265:2): "'What issues from your lips' is a positive commandment." And you know that there is no content to the statement, "what issues," alone. However the intention is that which I mentioned to you - to understand the simple meaning of the verse, that He commanded to do what one puts out with his mouth. And behold that this commandment is repeated; and that is His saying, "according to all that issues from his mouth shall he do" (Numbers 30:3). And the regulations of this commandment have already been explained in many places - in Shevuot, in the end of Menachot and in Tractate Kinnim as well. That is to say that the specifics of concerning oneself to do the thing that one has obligated oneself; and how one exempts oneself when he has a doubt about what he said, is found there. (See Parashat Ki Tetzeh; Mishneh Torah, Sefer Haflaah.)

If a householder makes a vow to יי or takes an oath imposing an obligation on himself, he shall not break his pledge; he must carry out all that has crossed his lips.

When a person takes vows in order to establish his character traits and correct his conduct, he is considered eager and praiseworthy. What is implied? If a person was a glutton and he [took a vow] forbidding meat for a year or two, a person was obsessed with wine and he [took a vow] forbidding himself from drinking wine for a prolonged period or he forbade himself from ever becoming intoxicated, a person would continually pursue illicit gain and was overexcited about wealth [took a vow] forbidding [accepting] presents or benefit from people in a particular country, similarly, a person who would be proud of his comely appearance and took a nazirite vow, or the like - all of these are paths in the service of God and concerning such vows and the like our Sages said: "Vows are a safeguard for restraint."

Although [taking vows] is an element of the service of God, a person should not take many vows involving prohibitions and should not habituate himself to taking them. Instead, he should abstain from those things from which one should abstain without taking a vow.

Our Sages stated: "Anyone who takes a vow is considered as having built a private altar." 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. With regard to vows involving the consecration of articles, it is a mitzvah to uphold them and not to ask for their absolution unless one is [financially] pressed, as [Psalms 116:14] states: "I will fulfill my vows to God."

A person who delays their vow, their notebook is opened.

One should be careful not to vow in any matter. Even tzedaka should not be vowed; rather, if one has what to give, they should immediately give it, and if not, they should not vow until they have it. And if one is part of the tzedaka-distribution process, it should be done while stating "This is not a vow!"

A person who says "I will learn this perek, and is afraid lest they become negligent in this matter, it is appropriate for them to vow in order to quicken themselves. And so, if one is afraid that their [evil] inclination will attack them and they will transgress any commandment of the negative commandments, or become negligent in the fulfilment of a positive commandment, it is a commandment for that person to vow in order to quicken themself.

[Although] a person has the intent to take an oath and resolves in his heart not to eat on that day or not to drink and has the intent for that activity to be forbidden for him by oath, [if] he does not actually make such a statement, he is permitted [to eat or drink], as [implied by Leviticus 5:4]: "expressing with his lips." [Implied is that] a person who takes an oath is not liable until he explicitly states the matter the oath concerns with his lips.

A person who takes an oath is not forbidden [to partake of] the entity which he forbade to himself until he makes a verbal statement to that effect and his statements must match his intent, as we explained with regard to oaths.
If, by contrast, one intended to take a nazirite vow and instead, vowed to bring a sacrifice, [intended to vow to bring] a sacrifice and instead, took a nazirite [vow], [intended to take] an oath and instead, [took] a vow, [intended to take] a vow and instead, [took] an oath, intended to say "figs" and instead, said "grapes," both are permitted to him. There is no vow.