Should the khums of the annual bonus vowed to Allah be paid, or should outstanding debts be settled with it first?

A vow does not hasten or delay anything from Allah's decree. Many scholars disliked making vows. However, if a Muslim makes a vow, he must fulfill it, unless it involves disobedience to Allah, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Him, let him not disobey Him." The vow must be fulfilled first, and then outstanding debts should be paid.