I am planning to cut some plugs with either XPS or EPS foam (whichever I can source cheaply) coated with a shell of fiberglass and body filler to make a polished finish to pull molds from. I plan to use a vinylester resin for the molds to helpfully reduce any shrinkage/warpage and am contemplating the possibility of vacuum bagging the mold for potentially a better surface finish/slightly stronger mold. Is there any benefit to vacuum bagging a mold vs just doing a wet layup, or is it more trouble and consumables than its worth? I don't really care about a lighter mold. I was just thinking that putting a vacuum on the mold would help reduce voids that need repaired before parts can be pulled and potentially help reduce warpage if excess resin is removed. 

Wet-lay vac bagging doesn't reduce void content compared to a properly wet-laid part. The idea is to give parts a better fibre:resin ratio, improving the strength to weight. For moulds as you said, this doesn't hugely matter. For moulds you actually generally want a bit of mass.

If you were infusing the moulds then that's a different story and well worth doing, but vac bagging no.