What is the ruling on vowing to donate to a person who is later found not to be deceased?

The fundamental principle regarding actions is that they are judged by the intention of the doer. The meanings of utterances are based on the intention of the one who utters them. This includes oaths and vows. If the one making a vow does not state their intention, then reference is made to the reason that prompted the vow, and the vow is then understood accordingly. A vow is a type of oath, so the statement of the one making a vow, "For Allah, I must do [such and such]," is equivalent to saying, "I swear by Allah that I will do [such and such]." Therefore, your vow was made contingent upon the death of the person for whom you vowed. Since this condition was not met, you are not obligated to fulfill this vow. However, if you choose to fulfill it, it is permissible and considered an act of charity.