I realize that if sought after, priests have authority to commute personal vows others make, but does this only apply to vows made by Catholics or personal vows made by a Protestant before joining the Catholic Church?
Actually, this is not a catechetical question. It is a canon law question.
You yourself would have the power to commute your own personal vows, per canon 1197 – presuming the commutation was equal to or greater than that which was originally vowed; what you would not have is the power to dispense yourself from your personal vows (or to commute to a lesser observance, also per canon 1197). These latter possibilities would require the intervention of someone else, named in canon 1196.
Here are the relevant canons for your situation from the Code of Canon Law:
*Can. 1196 In addition to the Roman Pontiff, the following can dispense from private vows for a just cause provided that a dispensation does not injure a right acquired by others:
1/ the local ordinary and the pastor with regard to all their subjects and even travelers;
2/ the superior of a religious institute or society of apostolic life if it is clerical and of pontifical right with regard to members, novices, and persons who live day and night in a house of the institute or society;
3/ those to whom the Apostolic See or the local ordinary has delegated the power of dispensing.
Can. 1197 The person who makes a private vow can commute the work promised by the vow into a better or equal good; however, one who has the power of dispensing according to the norm of ⇒ can. 1196 can commute it into a lesser good.
Can. 1198 Vows made before religious profession are suspended while the person who made the vow remains in the religious institute.*
The unique element you introduce to the equation regards canonical jurisdiction, since you are Protestant; I have never had a non-Catholic ask me to dispense them from a private vow. In any event, since you indicate you are in the process of becoming Catholic, the easiest solution is that certainly the pastor of the parish where you are coming into the Church would be able to grant the dispensation even before you join the Church since, even as a candidate/catechumen, he would thereby assuredly be your pastor from the perspective of Canon Law.
