Well, your prayers don't mean anything, either, and for the same reasons the vow meant nothing.
My prayers meant nothing because there was no fairy in the sky to listen to them.
So probably a good thing I didn't sacrifice anyone. I could have gotten in trouble for that.
But in the context of the story, the vow did mean something. Jephy made a vow that he didn't clearly think out. But he was so terrified of his sky fairy (which keep in mind, has spent the previous six books of the bible doing horrible things to both the Hebrews and their enemies) that he felt that the only option he had was to butcher his young daughter (probably less than 14), and then burn the parts on an altar. Because otherwise God might do horrible things to his people if he didn't follow through. He did horrible things to them for a lot less, after all.