Right now also in centrifugal because of high rpm as it is directly coupled to electric motor the pump creates partial vacuum which means the liquid will 
flow through vacuum hence the liquid will be recieved at the centrifugal pumps suction and as 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 air from the engine, thus reducing the air flow pressure build up created by blow because of Centrifugal Vacuum Pump combustion gases going past the piston rings in to the pan.