The story of Jephthah and his rash vow is an interesting one. Some people believe that Jephthah fulfilled his vow and killed his daughter as a sacrifice, while others believe he simply dedicated her to a life of service. After reading up on some of the arguments for and against both positions (this article and this one), I'm inclined to think he did the former.
Though God used Jephthah to accomplish His purposes, the Old Testament never refers to him as a godly man. In fact, if he was a godly man he would've known about God's provisions for rash vows and he would have made the necessary sacrifices to get out of his foolish vow. God also uses anyone He wishes, godly or ungodly, to accomplish His plans. He is mentioned in Hebrews 11, but just like Samson who slept with a prostitue and David who was an adulterer and murderer, he was still a sinner in need of a saviour. As abhorrent as the idea of child sacrifice is, it seems likely that that is indeed what occurred.
No matter what happened, it is a reminder that we are all guilty of making sinful choices - whether they are planned or impulsive - and we are all in desperate need of the saving grace Jesus came to provide.
Tomorrow's Bible In a Year Passage: Judges 13-15; Luke 6:27-49