Is a vow valid if one says, "I vow to Allah not to enter the guest room for two weeks after the exams," if the one making the vow was menstruating at the time of the vow?

Whether the vow was made during menstruation or purity does not affect it. As for vowing a permissible act (one for which there is no religious encouragement), scholars have differed on its validity. The majority of scholars hold that it is not valid and does not necessitate an expiation (kaffarah (definition)), citing the Prophet’s (peace be upon him) sayings: "Whoever vows to obey Allah, let him obey Him," and "There is no vow except in that which is sought for the sake of Allah," and the hadiths where the Prophet (peace be upon him) did not command an expiation for vows concerning permissible acts. However, Imam Ahmad held that it is valid and the one who vowed has the choice between fulfilling it or not, and its expiation is like that of an oath (yameen). The more cautious approach is to offer an expiation for an oath if the vow is not fulfilled, which is feeding ten poor people, or clothing them, or freeing a slave. If one cannot find any of these, then fasting for three days.