Objection 1. It would seem that a vow need not be always about a better good. A greater good is one that pertains to supererogation. But vows are not only about matters of supererogation, but also about matters of salvation: thus in Baptism men vow to renounce the devil and his pomps, and to keep the faith, as a gloss observes on Psalm 75:12, "Vow ye, and pay to the Lord your God"; and Jacob vowed (Genesis 28:21) that the Lord should be his God. Now this above all is necessary for salvation. Therefore vows are not only about a better good.
Objection 2. Further, Jephte is included among the saints (Hebrews 11:32). Yet he killed his innocent daughter on account of his vow (Judges 11). Since, then, the slaying of an innocent person is not a better good, but is in itself unlawful, it seems that a vow may be made not only about a better good, but also about something unlawful.
Objection 3. Further, things that tend to be harmful to the person, or that are quite useless, do not come under the head of a better good. Yet sometimes vows are made about immoderate vigils or fasts which tend to injure the person: and sometimes vows are about indifferent matters and such as are useful to no purpose. Therefore a vow is not always about a better good.
On the contrary, It is written (Deuteronomy 23:22): "If thou wilt not promise thou shalt be without sin."
I answer that, As stated above (Article 1), a vow is a promise made to God. Now a promise is about something that one does voluntarily for someone else: since it would be not a promise but a threat to say that one would do something against someone. On like manner it would be futile to promise anyone something unacceptable to him. Wherefore, as every sin is against God, and since no work is acceptable to God unless it be virtuous, it follows that nothing unlawful or indifferent, but only some act of virtue, should be the matter of a vow. But as a vow denotes a voluntary promise, while necessity excludes voluntariness, whatever is absolutely necessary, whether to be or not to be, can nowise be the matter of a vow. For it would be foolish to vow that one would die or that one would not fly.
On the other hand, if a thing be necessary. not absolutely but on the supposition of an end--for instance if salvation be unattainable without it--it may be the matter of a vow in so far as it is done voluntarily, but not in so far as there is a necessity for doing it. But that which is not necessary, neither absolutely, nor on the supposition of an end, is altogether voluntary, and therefore is most properly the matter of a vow. And this is said to be a greater good in comparison with that which is universally necessary for salvation. Therefore, properly speaking, a vow is said to be about a better good.
Reply to Objection 1. Renouncing the devil's pomps and keeping the faith of Christ are the matter of baptismal vows, in so far as these things are done voluntarily, although they are necessary for salvation. The same answer applies to Jacob's vow: although it may also be explained that Jacob vowed that he would have the Lord for his God, by giving Him a special form of worship to which he was not bound, for instance by offering tithes and so forth as mentioned further on in the same passage.
Reply to Objection 2. Certain things are good, whatever be their result; such are acts of virtue, and these can be, absolutely speaking, the matter of a vow: some are evil, whatever their result may be; as those things which are sins in themselves, and these can nowise be the matter of a vow: while some, considered in themselves, are good, and as such may be the matter of a vow, yet they may have an evil result, in which case the vow must not be kept. It was thus with the vow of Jephte, who as related in Judges 11:30-31, "made a vow to the Lord, saying: If Thou wilt deliver the children of Ammon into my hands, whosoever shall first come forth out of the doors of my house, and shall meet me when I return in peace... the same will I offer a holocaust to the Lord." For this could have an evil result if, as indeed happened, he were to be met by some animal which it would be unlawful to sacrifice, such as an ass or a human being. Hence Jerome says [Implicitly 1 Contra Jovin.: Comment. in Micheam vi, viii: Comment. in Jerem. vii. The quotation is from Peter Comestor, Hist. Scholast.]: "In vowing he was foolish, through lack of discretion, and in keeping his vow he was wicked." Yet it is premised (Judges 11:29) that "the Spirit of the Lord came upon him," because his faith and devotion, which moved him to make that vow, were from the Holy Ghost; and for this reason he is reckoned among the saints, as also by reason of the victory which he obtained, and because it is probable that he repented of his sinful deed, which nevertheless foreshadowed something good.
Reply to Objection 3. The mortification of one's own body, for instance by vigils and fasting, is not acceptable to God except in so far as it is an act of virtue; and this depends on its being done with due discretion, namely, that concupiscence be curbed without overburdening nature. on this condition such things may be the matter of a vow. Hence the Apostle after saying (Romans 12:1), "Present your bodies a living sacrifice, holy, pleasing to God," adds, "your reasonable service." Since, however, man is easily mistaken in judging of matters concerning himself, such vows as these are more fittingly kept or disregarded according to the judgment of a superior, yet so that, should a man find that without doubt he is seriously burdened by keeping such a vow, and should he be unable to appeal to his superior, he ought not to keep it. As to vows about vain and useless things they should be ridiculed rather than kept.
Source: Summa Theologica (New Advent)