the nice thing about the vacuum, though, is that it tends to expand, burst, and evacuate bubbles in the resin of the layup. Applying just external pressure would compress those bubbles and they would remain in place.
True, but the nice thing about large differentials in pressure (this includes the difference between atmospheric pressure and the applied vacuum, or atmospheric and the applied pressure) is that as long as the material is flowing, the air will rush to the flow front. In production comression molding uses this to get void free laminates. Puddle the resin in the center and as you apply pressure the resin flows out and takes the air with it. Air voids (short circuits)under vacuum will not "move" either without flow.
