If I'm not mistaken fences prevent the spanwise flow across the wing,
delaying the onset of a stall. For the airplanes that we typically fly
the wing stalls from the root first then spanwise to the tips, the
fences help to reduce this. Vortex generators on the other hand do
exactly what their name imply, create vortices that re-energize the
boundary layer on the wing. Slots also serve a similar purpose, except
that higher energy flow from beneath the wing is allowed to travel to
the upper surface and re-energize the boundary layer.

Dave