I’m going to assume, for the purposes of brevity, that we’re working with a finished, female mold, which is coated and sealed to the point of being airtight. It is easy to vacuum bag over a male mold, as well; the technique is identical. I’ll also assume that the mold has a peripheral flange (a sealed surface, not part of the functional mold surface, running the perimeter of the mold). If you’re working on a mold with no flange, vacuum bagging is possible, but you will still need an area, continuous with the mold-laminating surface, to seal to. If your mold is not airtight, I’ll make a note about that…