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