She vowed to fast for a month, should she make up the days when she was menstruating?

If a woman vows to fast
for a month, then she should make up the days when she was menstruating,
just as she makes up the days when she menstruates during the month of
Ramadaan. 

See al-Mughni by
Ibn Qudaamah, 10/86. 

The Standing Committee
for Issuing Fatwas was asked this question and replied: 

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 the number of days of that
month when you did not fast. 

And Allaah is the Source
of strength. May Allaah send blessings and peace upon our Prophet Muhammad
and his family and companions. 

Fataawa al-Lajnah
al-Daa’imah, 23/336.