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