First, it sounds like V was trying to help, overcommited and got bit. It wouldn't be an issue if you went with J, but you're not going to.
From a legal standpoint, I can't help. I suspect there's no way anyone can know if you used his .step files or not, because you could easily claim to have gotten your prototype rescanned elsewhere, but I can't say for sure.
From an ethical standpoint, you need to make V whole. Whether he agreed to free, or to compensate J, it's clear he's taken a bullet for you.
If you really need to use that .step file, negotiate a deal with V in writing. Pay him his $600, agree to installments, or agree to a reduced settlement. Don't let $600 stop you from doing the right thing. Nobody should get burned trying to help.