§1. A vow, that is, a deliberate and free promise made to God about a possible and better good, must be fulfilled by reason of the virtue of religion.
§2. Unless they are prohibited by law, all who possess suitable use of reason are capable of making a vow.
§3. A vow made out of grave and unjust fear or malice is null by the law itself.
Source: Code of Canon Law (Vatican.va)