Is expiation obligatory for one who made a covenant with Allah to perform the night vigil (qiyam al-layl) but was overcome by sleep on some nights?

If you uttered that you covenant Allah to regularly perform the night vigil (Qiyam al-Layl), this is considered a vow or an oath according to the preponderant opinion of scholars, and you are obligated to offer an expiation for an oath (kaffarat yamin), based on the Prophet's (peace be upon him) saying: "The expiation for a vow is the expiation for an oath." The expiation for an oath is feeding ten poor people, or clothing them, or emancipating a slave. If you cannot find any of these, then fasting for three days.