Ok, so more theory when I get the time, but basically..
The bottom roller is driven by a motor causing it to spin.
The bottom roller is make from PVC which is towards the bottom (-ve end) of the Triboelectric scale
The belt is made from Latex (Rubber) which is near the middle of the scale. As the belt rolls around the lower roller Nylon and Latex come together and are separated The Nylon roller (+ve end of the Triboelectric scale) will become positively charged and the inside of the belt negatively charged. The outside of the belt is therefore positively charged by electrostatic induction.
The belt carries this charge up to the top of the VDG. The charge on the belt is taken off by a metal comb, and the belt continues around the roller, but in this case the roller is made from PVC (-ve end) of the Triboelectric scale. So the belt carries a negative charge back towards the bottom roller.
Another comb on the bottom roller removes this -ve charge to earth. The belt continues around and the cycle continues.