I'm trying to find a way to replicate something using vacuum forming around the shape of the original object, and then layering some sort of resin to make a shell. The problem is that the object I want to replicate is expensive so I definitely don't want to risk damaging it by vac-forming hot plastic around it and suctioning air out and risk damaging it with any kind of pressure.

I want to find a way to make a cast to go over the original object, fill the cast with plaster for a ~1:1 model, touch up the shape a bit, and THEN vacuum form a mold over the plaster. (all this to preserve the condition of the original object.) After that I'll layer some sort of resin around the mold to make a shell. If there is some sort of material I can make the mold of without having to do the 2nd and 3rd steps I'm planning, it would be a great bonus and I would shower you with cookies and thanks.

My main question is this: What would be a good thing to make the original cast from? (Something that won't stick easily to the original surface or is known for being very gentle to plastic surfaces.)

If it's that rare and valuable, it might be better to just have it 3D scanned and then CNC carve a mold.

You might be able to try using the methods that people use to create 1:1 resin molds of Warhammer 40,000 molds. Depends on size, etc.
If you don't mind my asking, what is this mystical object?