Yes. One needs a seal on both sides of the layup. There is a whole thread here about "strechalon" molds. Those molds can kill details in soft foam molds. Another method of cheap one off molding is to harden the foam surface with something that isn't porous like a layer of epoxy or filler, paint, whatever. Molding is tough depending on the level of detail like joggles and rebates for covers, etc... For simple parts there is no reason to vacuum a layer over a mold. You can just put the layup in a bag and use both halves of the foam to hold it in position while it cures. You can get good compaction and resin fractions this way but you can also get wrinkles and bunching. So it has it's uses but it certainly isn't a production technique. What I have do quite often to get a prototype skin is to make a foam mold by hot wiring and carving, cnc cutting sections of the mold. Then you can cut mylar or thin aluminum to line the mold. Then you wet layup onto those shiny surfaces and put the whole layup on one layer of bagging material and put peal ply and wicking material on top of that and another layer of bagging material on top of that. Then put the other half of the hot wired foam on top of that, weigh it down and pull the vacuum. Even multi tapered parts or parts that have other than flat wrapped can be done to the point where you have eliminated say 90% of the filling and sanding. You are only limited by your imagination. A mold as one carved part is something that stops people from being clever. A mold can be made up of many pieces carved separately using different techniques. Again not for production work.