Must one make up a vowed fast before fasting the six days of Shawwal, or vice versa?

Fulfilling a vow is obligatory. Whoever vows to perform an act of obedience must fulfill it, such as fasting a vowed fast, which must be fulfilled. This is unlike fasting the six days of Shawwal, which is not obligatory. A specific vow, such as fasting a particular Thursday, is forbidden to delay, and delaying it incurs an expiation of an oath, in addition to the obligation to fast it. As for an absolute, unspecified vow, fasting is obligatory, but there is no sin in delaying it, and no expiation is required.