The only cheap way I know of to vac-form this would only result in a one time cast, because I can't see it lasting for more then one, maybe two.
:/ If I can talk to my guy whos going to be forming my arm pieces, but even then, I don't think his vac-former would even be large enough for the chest. The way he does his bucks, is that after he sculpts it, he fills the interior of the buck, and the outside with this really dense foam, from smoothon I think, and sands the excess, molds it, and then makes a solid cast in a high strength resin
The foam itself goes for about $90, then pluse the silicone, could be about $150, just for the one mold...so yea..