There are cases when you are allowed to annul a vow on Shabbos. The most common case is for the needs of Shabbos or another mitzvah. If it can wait, it is forbidden either because of tircha or because of. “Mim’tzo cheftz’cha”. (ShA 1 – MB 1 – ShaH 1)
Shabbos needs include eating meat and drinking wine or any other food which is commonly used for oneg Shabbos. You are also allowed to nullify a vow which would stop you from wearing suitable clothing for Shabbos or from sleeping. (MB 2)