Objection 1. It would seem that the authority of a prelate is not required for the commutation or dispensation of a vow. A person may enter religion without the authority of a superior prelate. Now by entering religion one is absolved from the vows he made in the world, even from the vow of making a pilgrimage to the Holy Land [Cap. Scripturae, de Voto et Voti redempt.]. Therefore the commutation or dispensation of a vow is possible without the authority of a superior prelate.
Objection 2. Further, to dispense anyone from a vow seems to consist in deciding in what circumstances he need not keep that vow. But if the prelate is at fault in his decision, the person who took the vow does not seem to be absolved from his vow, since no prelate can grant a dispensation contrary to the divine precept about keeping one's vows, as stated above (10, ad 2; 11). Likewise, when anyone rightly determines of his own authority that in his case a vow is not to be kept, he would seem not to be bound; since a vow need not be kept if it have an evil result (2, ad 2). Therefore the Authority of a prelate is not required that one may be dispensed from a vow.
Objection 3. Further, if it belongs to a prelate's power to grant dispensations from vows, on the same count it is competent to all prelates, but it does not belong to all to dispense from every vow. Therefore it does not belong to the power of a prelate to dispense from vows.
On the contrary, A vow binds one to do something, even as a law does. Now the superior's authority is requisite for a dispensation from a precept of the law, as stated above (I-II, 96, 6; I-II, 97, 4). Therefore it is likewise required in a dispensation from a vow.
I answer that, As stated above (1 and 2), a vow is a promise made to God about something acceptable to Him. Now if you promise something to anyone it depends on his decision whether he accept what you promise. Again in the Church a prelate stands in God's place. Therefore a commutation or dispensation of vows requires the authority of a prelate who in God's stead declares what is acceptable to God, according to 2 Corinthians 2:10: "For... have pardoned... for your sakes... in the person of Christ." And he says significantly "for your sakes," since whenever we ask a prelate for a dispensation we should do so to honor Christ in Whose person he dispenses, or to promote the interests of the Church which is His Body.
Reply to Objection 1. All other vows are about some particular works, whereas by the religious life a man consecrates his whole life to God's service. Now the particular is included in the universal, wherefore a Decretal [Cap. Scripturae, de Voto et Voti redempt.] says that "a man is not deemed a vow-breaker if he exchange a temporal service for the perpetual service of religion." And yet a man who enters religion is not bound to fulfil the vows, whether of fasting or of praying or the like, which he made when in the world, because by entering religion he dies to his former life, and it is unsuitable to the religious life that each one should have his own observances, and because the burden of religion is onerous enough without requiring the addition of other burdens.
Reply to Objection 2. Some have held that prelates can dispense from vows at their will, for the reason that every vow supposes as a condition that the superior prelate be willing; thus it was stated above (Article 8) that the vow of a subject, e.g. of a slave or a son, supposes this condition, if "the father or master consent," or "does not dissent." And thus a subject might break his vow without any remorse of conscience, whenever his superior tells him to.
But this opinion is based on a false supposition: because a spiritual prelate being, not a master, but a dispenser, his power is given "unto edification, not for destruction" (2 Corinthians 10:8), and consequently, just as he cannot command that which is in itself displeasing to God, namely, sin, so neither can he forbid what is in itself pleasing to God, namely, works of virtue. Therefore absolutely speaking man can vow them. But it does belong to a prelate to decide what is the more virtuous and the more acceptable to God. Consequently in matters presenting no difficulty, the prelate's dispensation would not excuse one from sin: for instance, if a prelate were to dispense a person from a vow to enter the religious life, without any apparent cause to prevent him from fulfilling his vow. But if some cause were to appear, giving rise, at least, to doubt, he could hold to the prelate's decision whether of commutation or of dispensation. He could not, however, follow his own judgment in the matter, because he does not stand in the place of God; except perhaps in the case when the thing he has vowed is clearly unlawful, and he is unable to have recourse to the prelate.
Reply to Objection 3. Since the Sovereign Pontiff holds the place of Christ throughout the whole Church, he exercises absolute power of dispensing from all vows that admit of dispensation. To other and inferior prelates is the power committed of dispensing from those vows that are commonly made and frequently require dispensation, in order that men may easily have recourse to someone; such are the vows of pilgrimage (Cap. de Peregin., de Voto et Voti redempt.), fasting and the like, and of pilgrimage to the Holy Land, are reserved to the Sovereign Pontiff [Cap. Ex multa].
Source: Summa Theologica (New Advent)