Hi andy
A compressor is controlled by a pressure switch,Tank empty switch it on and compressor will run filling the tank, when it's upto the set pressure the pressure switch kills the supply to the motor, as you use air the tank pressure drops below the switches set point,the contacts close and power is supplied to the motor increasing the pressure.
If using as a vac pump all you need to do is open the outlet valve on your tank so the switch will never see the cutout pressure.
There is usually a filter of some sort on the inlet, use this connection for pulling a vacuum.
Regards
Mike