Hachana (preparing for post-Shabbos needs). The Sages forbade any extra effort or preparation (even minor) during Shabbos or Yom Tov on behalf of a following weekday. This holds true even if the effort involves no melacha or (other) Rabbinic ordinance whatsoever. This prohibition, called ‘hachana’ (lit. preparation) was meant to preserve the serenity and restful character of Shabbos and Yom Tov… In fact, even a verbal expression that effects a halachic condition (for a post-Shabbos need) may also be considered hachana, because even this is considered a sort of “bother” for post-Shabbos needs.
Example: Releasing a halachic ‘neder’ (vow - Heb. ‘hataras nedarim’) on Shabbos: The words used by the Rav to release the neder is effectively a verbal expression that causes a halachic change (and is prohibited when done for the benefit of the following weekday.)
Ed: The clear implication of the above halacha is that if the annulment of the vow enhances one’s ability to observe Shabbos (e.g. he made a vow which prohibits him from eating certain kinds of foods one would typically eat on Shabbos, and the annulment will permit him to eat it once again), then the “toil” involved is justifiable, and the annulment is permitted on Shabbos