Depends on what you are wanting to do. Stabilization ie impregnating wood with resin requires a good vacuum. Casting polymers with wood needs pressure to force the resin into the nooks and crannys. Both can be used in a single casting, first vacuum to debubble the resin, then pressure to push the resin into the piece.
Worthless wood casting is often done by stabilizing the wood first then pressure casting resin into the blank.