When you shall vow a vow unto the LORD your God, you shall not delay to pay it: for the LORD your God will surely require it of you; and it would be sin to you.
All Commentaries on Deuteronomy 23:21 Go To Deuteronomy 23
Ambrose of Milan
AD 397
A vow is a request for a benefit from God with a promise to give something in return. Hence, when you have obtained what you sought, it would be an ungrateful act to delay what you have promised. .