All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companion.
A vow is an act of obedience that Allah enjoined fulfilling where He said (What means):" Then let them perform their vows.” [Al-Hajj/29]. He, The Almighty, Has also Praised those who fulfill their vows for He Said ( What means): ”They perform (their) vows, and they fear a Day whose evil flies far and wide. [Al-Insan/7]. Further, making a vow to fast every Monday, Tuesday and the white days as long as being alive renders this act {fasting} an obligation, entails the same reward of obligatory fast and is not considered a voluntary act of worship. Therefore, you (Questioner) are obliged to fulfill the vow you made to the best of your ability. This is because through this vow fast has become an obligation on you and you will remain accountable for it so long as your are able to fast.
Furthermore, if you can`t endure fast due to a chronic disease or aging, then you are obliged to pay fasting ransom; a Mudd(600gr) of the common food of the country for each broken fast since Allah, the Exalted, Said (What means): "For those who can do it (With hardship), is a ransom, the feeding of one that is indigent.” [Al-Baqarah/184]. However, if you couldn’t pay the ransom, then you will remain accountable to fulfill your vow, either by fasting or paying the ransom.
According to some jurists, you are permitted to pay an oath expiation (Feeding 10 poor Muslims; a Mudd for each) based on the Hadith in which the Prophet (PBUH) said (What means):" Its expiation is the expiation for an oath.” [Al-Tirmithi & Abu Dawood]. Yet, in order to be on the safe side, there is no harm in adopting the latter opinion.
In conclusion, you won’t be accountable as regards the fasts you missed during menses because this isn’t considered a part of the vow that you have to fulfill. Besides, fasting while having menses isn’t valid in the first place.
In this regard, Al-Imam Al-Ansari stated: "If a person makes a vow to fast each Monday; then he isn’t permitted to include Ramadan’s Mondays as a part of his oath, and the same applies on Mondays of the two Eids, Tashriq days, menses and confinement (Postnatal bleeding).” [Asnaa’ Al-Matalib vol.1/pp.583]. And Allah Knows Best.
