When I first heard of the oil starvation problems with motors running vacuum pumps I thought it was due to tight of clearances on the wrist pins to rod and pistons due to most pin oiling being splash oiling.
The oil pump is a suction pump also but it forces liquid through a restrictor, the engine oiling system and has a high pressure bypass system built into the pump so the motor will have a maximum oil pressure at the higher RPM. The oil pump has to battle the negative pressures above the oil level to suck oil into it, or does it
Using a vacuum pump does allow thinner rings to seal better which allows the motor to make more power with less oil contamination in the combustion chambers
As well as stop some of those pesky tiny oil leaks