If you are using a fat sack and a reversible pump, connect the pump at the bottom of the sack,not the top, otherwise it won't empty properly.
Regarding the vent, you will need one to ensure you don't pop your sack due to too much water pressure. My suggestion is you also add a check valve in the vent to allow air out, but not back in, that way the pump will suck the sack dry. Without a check valve, you find that the pump is sucking air rather than water out of the sack when it gets towards empty.
If you are using a rigid tank rather than a sack, do not use a check valve as you won't be able to suck the water out unless air can get in to replace it