I found out that Jewish and Christian historians, theologians, and apologists almost unanimously believed up until about 1000 AD that Jephthah had sacrificed his daughter. Christian apologists believing this included Origen, St. Jerome, St. Augustine, St. Thomas Aquinas, Martin Luther, and John Calvin.
I had an email exchange with a Christian preacher about this topic and he said that Jephthah wouldn't have been included in the Hebrews 11 "hall of faith" if he had sacrificed his daughter. I pointed out that David was included in the Hebrews 11 list despite his adultery and murder.
The comparison to David got me thinking...perhaps it could be argued that the sacrifice was allowed to go through because it was a result of trying to barter with the Lord. In a similar manner, the Bible says in 2 Samuel that the son born to David via Bathsheba died as a result of his sins of murder and adultery. Furthermore, there is a clear distinction in why God stopped the killing of Isaac, but not Jephthah's daughter. Abraham and Isaac was a test of faith that was initiated by God. Jephthah's situation, on the other hand, was initiated on his own accord and I suspect it would be considered a sin to make a deal with God.