Well according to the thurston james book of mold making and casting, dosnt say exactly for making a plaster mold from plastic, but casting in materials of polyurethane plastics, you can use lacquer, PVA, talc, and wax. Hope this helps.
I would use vaseline, I have made many molds from plaster and used it without any problems.
I make the plaster mold, then coat the inside with a thinned down layer of plaster to fill in any bubble holes, then lightly apply the vasoline, and never had any thing stick when making the cast.
Vaseline is in the same catagory as duct tape, it has thousands of uses.
I've used Vaseline as mold release and also used PAM nonstick spray.
Depends on what your doing. Plaster doesn't really stick to some plastics so you might not even need it but better to be safe than sorry.
hope this helps.
Wayne