Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister, we are greatly pleased to receive your question which shows the confidence you place in us. May Allah reward you abundantly for your interest in knowing the teachings of Islam!
If a woman vows to fast for a month, then she should make up for the days of fasting when she was menstruating, just as she makes up the days when she menstruates during the month of Ramadan.
Responding to your question, The Standing Committee for Islamic Research and Ifta’ states:
You have to fast the month which you vowed to fast, and if you break the fast during part of it because of menstruation, then you have to make up for the number of days of that month when you did not fast.
Allah Almighty knows best.