If someone makes a vow to God to do a certain thing, and then later rashly promises not to do it, which vow would be standing in God's eyes? They were both vows, but the vows contradict each other, so which one should the vower keep?
I agree with Jason. Why would someone think that they could 'vow' their way out of a 'vow'? If you made a vow and didn't keep it then you haven't kept your vow, period. You might keep the second vow, but you still haven't kept the first and so you haven't kept your vow. If someone were to make two contradictory or otherwise opposing vows, then one vow isn't going to be kept.