hottie,
I believe that generally speaking a vortex generator placed on an engine nacelle is there to combat a destructive vortex produced by the nacelle in the slower flight area.
The vortex gererator will produce a vortex which will redirect the destructive one essentially cancelling it out.Some may redirect it to an area where it may even become usefull....
again, this is generally speaking.
try searching tech log... i recall reading some information regarding this subject there.