Today also in centrifugal due to high rpm as it is directly coupled to motor the pump creates partial vacuum this means the liquid will certainly flow through vacuum Centrifugal Vacuum Pump therefore the liquid will end up being recieved 
at the centrifugal pumps suction and because the impeller rotates the liquid will be displaced away forward with a particular energy.
A vacuum pump gets the inlet hooked up to one or both valve covers, sometimes the valley pan. It SUCKS the air from the engine, hence reducing the air pressure build up created by blow because of combustion gases going past the piston rings in to the pan.