Hi John,
I checking through some translations, here are a few to consider.
Jdg 11:31 then shall it be, that, whosoever cometh forth out of the doors of my house, to meet me, when I return successful, from the sons of Ammon, shall belong unto Yahweh, and I will offer him up, as an ascending-sacrifice. (Rotherham)
Jdg 11:39 and it comes to pass at the end of two months that she turns back unto her father, and he does to her his vow which he has vowed, and she knew not a man; and it is a statute in Israel.
v. 40 from time to time the daughters of Israel go to talk to the daughter of Jephthah the Gileadite, four days in a year. (CLV)
What is lost in translation can obviously make a big difference. Maybe these translations show it is not as cut and dry as the others make it out to be. It's really hard to believe that it was meant as a human sacrifices, which God makes well known as an abomination in His sight and is prohibited in OT law, with a penalty of death (Lev 18:21; Lev 20:2-5; Deu 12:31; Deu 18:10). So maybe the kind of "offering" was determined by what came from his house first?
mercy, peace and love
Kat