I'm no Biblical scholar, Jennifer, but she now becomes the Lord's. This I assume means that she essentially becomes a nun - hence her bewailing her virginity - and this ends Jephthah's ancestral line. Not a good thing for someone with his ambition.
This all makes sense considering that human sacrifice at that time was not pleasing to God, so He - knowing the outcome - wouldn't have agreed to the deal to begin with.
What amazes me more than anything are all the deals OT God has to make in order to get his chosen people to behave, then they do it so poorly.