Canon Law seems to suggest not making another vow, but substituting a more reasonable good for the vow.
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.
vatican.va/archive/ENG1104/__P4E.HTM
Other passages from Canon law regarding a private vow:
Can. 1193 By its nature a vow obliges only the person who makes it.
Can. 1194 A vow ceases by the lapse of the time designated to fulfill the obligation, by a substantial change of the matter promised, by the absence of a condition on which the vow depends, by the absence of the purpose of the vow, by dispensation, or by commutation.
Can. 1195 The person who has power over the matter of the vow can suspend the obligation of the vow for as long a time as the fulfillment of the vow brings disadvantage to that person.
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;