Right now also in centrifugal because of high rpm as it is directly coupled to motor the pump creates partial vacuum which means the liquid can flow through vacuum hence the liquid will be recieved at the centrifugal pumps suction and because the impeller rotates the liquid will be displaced away forward with a specific energy.
A vacuum pump has the inlet hooked up to 1 or both valve covers, 
sometimes the valley pan. It SUCKS the atmosphere from the engine, thus reducing the air flow pressure Centrifugal Vacuum Pump build-up created by blow due to combustion gases going past the piston rings in to the pan.