Jephthah vowed a vow to Yahweh, and said, If you will indeed deliver the children of Ammon into my hand,
This, along with Jephthah's honoring the vow after his victory, is the big problem with this entire narrative.  Our confidence in the ASV is behind our interpretation of this passage.  In spite of the fact that a marginal reading of "whosoever cometh forth ... to meet me" is given, our translators rendered the word as whatsoever, indicating that Jephthah certainly did not anticipate that a human being would be the first to meet him.  This meaning is emphasized by the impersonal pronouns, "it shall be Jehovah's," and, "I will offer it for a burnt-offering."