Short Answer:  A VDG is a charge pump.  One or both rollers become charged
through contact with the belt.  One roller sucks electric charges from its
adjacent metal comb and onto the belt.  The other roller pushes electric
charges from the belt and onto the adjacent comb.  When the belt is
cranked along, the device sucks charges in at one end and spits them
out at the other.

Longer Answer: 
Everyday objects are made up of equal amounts of positive electric charge
and negative electric charge in almost perfect balance.  We might say that
ordinary matter is made up of "cancelled-out electricity."  A VDG machine
takes the mixed-up positive and negative charges of matter, sorts them
out, then pulls them far apart from each other.  A VDG is a "charge
uncanceller" or "charge separator."  The machine moves charge
continuously, so it also acts as a mechanical charge pump.

A VDG machine contains a flat circular belt running on a pair of rollers,
conveyor-belt style.  The belt material must be an insulator;  rubber or
plastic for example.  A metal "comb" is placed adjacent to each roller,
with "teeth" pointing toward the belt surface.  At each end of the belt,
each roller and comb is enclosed inside a hollow metal box or hollow
sphere.  Each hollow box or sphere must be electrically connected to the
metal comb inside.  One of the rollers is spun by an electric motor so
that the belt moves, and the other roller spins too.

To create a buildup of separated charge, the machine pulls one type of
charge ( either pos. or neg. ) out of one comb and places it onto the
belt's surface.  The belt transports it to the far end of the machine.
The electric charge is then pushed off the belt surface and onto the other
metal comb, where it is sucked to the outside of the metal sphere.  As the
charge-transport process continues, the voltage (electric potential)
between the two ends of the generator grows and grows.