Fiberglass cloth will print through and do all sorts of irregularities if messed with when the resin is green enough to accomodate a 3-D shape. You can use stretched membranes to improve the finish on male shapes, but I've not heard of a vacuum bag being used to give a molded part two good surfaces.
For putting a nice skin on foam, here's a new spray-on coating:
Industrial Polymers Corporation-StyroSpray
I've also had good success producing mold bucks (one-use master shapes) from foam covered with easy-sanding wall filler and then painted. I had a mix of latex paint, spackle, and microbaloons that sanded as easy as regular 2-lb insulation foam, but could get a hard, shiny surface by being coated with either PVA or Shellac and polished as usual.