Is a fast intended as a vow, but not conditioned upon the occurrence of something, considered an obligatory vow or a voluntary act that permits breaking the fast?

If you utter the words of a vow, you must fulfill it by fasting the days between the completion of your make-up fasts and Ramadan, because the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Allah, let him not disobey Him." It is permissible to forgo fasting on the day of doubt (Yawm al-Shakk), but you are obligated to fast the days before it due to the vow. If you fail to fulfill this vow, you are required to make up the fasting of those days and offer an expiation for an oath.