The vow you had made falls under the category of
vowing to do a Mubah (permissible) act. You have the choice either to fulfill the vow or offer Kaffarah (expiation) for not fulfilling it. The Kaffarah involves feeding ten Miskins (needy persons), (Part No. 23; Page No. 214) each Miskin receiving half a Sa` (1 Sa`=3 kg. Approx.) of food, or providing them with clothing, or emancipating a believing slave. If none of these is affordable, you must observe Sawm (Fast) for three days.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.