Generally speaking the contrast is controled by the voltage. Power source has VDD, GND, VO, VEE and the VO is the essential voltage that controls the contrast . We usually suggest our customer to use 20K VR and put between VDD and GND, or between VDD and VEE (as in the figure below).Vo is the differential voltage.

The Vo is the differential voltage between VDD and GND.	When using VEE, the VO is the differrential voltage between VDD and the VEE.