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