If a woman vows to fast for a month, then she should make up for the days she missed during her mensural cycle, just as she makes up for the missed days when she menstruates during the month of Ramadan.
A similar response was given on this issue by The Standing Committee for Islamic Research and Ifta’ when a question was asked. It goes as thus:
You have to fast the month which you vowed to fast, and if you break the fast within that month because of menstruation, then you have to make up for the number of days of that month you did not fast later on.