Objection 1. It would seem that it is not expedient to take vows. It is not expedient to anyone to deprive himself of the good that God has given him. Now one of the greatest goods that God has given man is liberty whereof he seems to be deprived by the necessity implicated in a vow. Therefore it would seem inexpedient for man to take vows.
Objection 2. Further, no one should expose himself to danger. But whoever takes a vow exposes himself to danger, since that which, before taking a vow, he could omit without danger, becomes a source of danger to him if he should not fulfil it after taking the vow. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Since thou hast vowed, thou hast bound thyself, thou canst not do otherwise. If thou dost not what thou hast vowed thou wilt not be as thou wouldst have been hadst thou not vowed. For then thou wouldst have been less great, not less good: whereas now if thou breakest faith with God (which God forbid) thou art the more unhappy, as thou wouldst have been happier, hadst thou kept thy vow." Therefore it is not expedient to take vows.
Objection 3. Further, the Apostle says (1 Corinthians 4:16): "Be ye followers of me, as I also am of Christ." But we do not read that either Christ or the Apostles took any vows. Therefore it would seem inexpedient to take vows.
On the contrary, It is written (Psalm 75:12): "Vow ye and pay to the Lord your God."
I answer that, As stated above (1 and 2), a vow is a promise made to God. Now one makes a promise to a man under one aspect, and to God under another. Because we promise something to a man for his own profit; since it profits him that we should be of service to him, and that we should at first assure him of the future fulfilment of that service: whereas we make promises to God not for His but for our own profit. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "He is a kind and not a needy exactor, for he does not grow rich on our payments, but makes those who pay Him grow rich in Him." And just as what we give God is useful not to Him but to us, since "what is given Him is added to the giver," as Augustine says (Ep. cxxvii, ad Arment. et Paulin.), so also a promise whereby we vow something to God, does not conduce to His profit, nor does He need to be assured by us, but it conduces to our profit, in so far as by vowing we fix our wills immovably on that which it is expedient to do. Hence it is expedient to take vows.
Reply to Objection 1. Even as one's liberty is not lessened by one being unable to sin, so, too, the necessity resulting from a will firmly fixed to good does not lessen the liberty, as instanced in God and the blessed. Such is the necessity implied by a vow, bearing a certain resemblance to the confirmation of the blessed. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.) that "happy is the necessity that compels us to do the better things."
Reply to Objection 2. When danger arises from the deed itself, this deed is not expedient, for instance that one cross a river by a tottering bridge: but if the danger arise through man's failure in the deed, the latter does not cease to be expedient: thus it is expedient to mount on horseback, though there be the danger of a fall from the horse: else it would behoove one to desist from all good things, that may become dangerous accidentally. Wherefore it is written (Ecclesiastes 11:4): "He that observeth the wind shall not sow, and he that considereth the clouds shall never reap." Now a man incurs danger, not from the vow itself, but from his fault, when he changes his mind by breaking his vow. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Repent not of thy vow: thou shouldst rather rejoice that thou canst no longer do what thou mightest lawfully have done to thy detriment."
Reply to Objection 3. It was incompetent for Christ, by His very nature, to take a vow, both because He was God, and because, as man, His will was firmly fixed on the good, since He was a "comprehensor." By a kind of similitude, however, He is represented as saying (Psalm 21:26): "I will pay my vows in the sight of them that fear Him," when He is speaking of His body, which is the Church.
The apostles are understood to have vowed things pertaining to the state of perfection when "they left all things and followed Christ."
Source: Summa Theologica (New Advent)