Regarding the crankcase. While the pistons are going up and down they are disp[lacing volume on the top of the piston but it cannot nbe any different on the bottom of the piston. The pistons are moving a lot of air in the crankcase and it takes energy to compress that air. Then when it expands back all of the energy doesn't come back. There's also the fact that air has to rush around the spinning crank, connecting rods, etc. So by adding cubic inches of volume to the crankcase, the air doesn't have to be compressed that much because the piston is working with a larger volume. The vacuum pump attacks the problem another way by making the crankcase air much less dense. It doesn't take as much energy to move around this less dense air. The vacuum might also assist in ring loading but I'm not sure how big the effect is.
One problem will pop up with decreased crankcase pressure, and that's technically called Net Positive Suction Head (NPSH). The oil pump doesn't suck oil in, it's the absolute pressure of the atmosphere that pushes the oil into the pump inlet. When you decrease the absolute pressure in the crankcase, it reduces the pressure available to push the oil into the oil pump inlet. I have a friend who had problems with oil pressure at the end of the track because his vacuum pump was so effective.
