What is the ruling on one who vowed to fast for a month or three months, then fasted for 12 days and stopped? What is incumbent upon him, and is it permissible for him to resume fasting in Rajab?

You are obligated to fulfill the vow when it becomes easy for you to do so, and to strive to ascertain the vowed number. If the number is not clear, then you must fast the greater number, according to many scholars, to discharge your responsibility with certainty, or the lesser number, according to some.

If you intended to fast consecutively, then it is binding upon you; otherwise, consecutive fasting is not required according to the majority of scholars, and you may build upon what you have already fasted.

If the vow was for an unspecified month, you may begin at any time. However, if the vow was for a specific month and its time has passed, then you must make up for it according to the majority of scholars. In addition to making it up, the Hanbalis require an expiation for an oath (kaffarat yameen) for delaying the vow, while the Malikis do not see anything due if the delay was for an excuse.