I know that some friends use PVC as molds, and I think one mentioned using mineral oil for the “lining”, and another said she used curled freezer paper to line the PVC.
I haven’t tried it (don’t like round soaps) but I think I’d not use anything, and then just gently warm the PVC and soap when it’s time to unmold and then push it out from one end to the other.
The freezer paper, rolled into a tube and put inside and allowed to expand is probably the most reasonable way to do it though!