Is it obligatory to fulfill a vow of performing the night vigil (Qiyam al-Layl) at a specific time (3:0 AM), or is it permissible to make it up at another time before Fajr for convenience? And is a vow a debt that must be paid?

Whoever vows an act of obedience must fulfill it, based on the Almighty's saying: "And let them fulfill their vows" (22:29), and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." Designating the third hour for performing qiyam is not considered an act of qurbah (drawing closer to Allah) unless it coincides with the last third of the night; in that case, it is obligatory to fulfill it. If it is not an act of qurbah, then it is considered a permissible vow, and its owner has the choice between fulfilling it or offering an expiation for an oath (kaffarat yamin). It is more prudent, if he does not perform qiyam at that time, to offer kaffarat yamin or perform qiyam for the same duration during the last sixth of the night before dawn.