I'm sorry, well I don't know much about vacuumformed molds. But yes, I am using it as a negative mold. Would I have to brush vaseline on the inside? Or use some mold release? Any advice would help me out a ton, thanks so much guys.
It's been my experience that you get much better pulls from a tough hollow mold, thus use either fiberglass or a high temp resin for your positives. With that said, both would require a release agent of some sort (i.e. PVA for fiberglass / silicone spray for resin).
It would help to know what your plans are for making the mold so we can better assist you.