Now also in centrifugal due to high rpm as it is directly coupled to engine the pump creates partial 
vacuum which means the liquid can flow through vacuum hence the liquid will be recieved in the centrifugal pumps suction and Centrifugal Vacuum Pump because the impeller rotates the fluid will be displaced aside 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 surroundings from the engine, therefore reducing the atmosphere pressure build up created by blow due to combustion gases going at night piston rings in to the pan.