[aerohead]

Originally Posted by cyclopathic

Found this article:
Aerodynamic Effects of Rear Spoiler and Vortex Generators on Passenger Cars (PDF Download Available)

There is more in article, but it is clear that at least in combination with rear spoilers VGs help to reduce aerodrag.

It is only clear in the context of the specific vehicle investigated (Mitsubishi Lancer?)
Since VGs were intended for notchback type cars,the caveat to VGs would be,if you have a notchback car very much like the Lancer,then there is an opportunity to see a very small drag reduction with the addition of properly-sized and positioned VGs.
No broad and sweeping claims can be made to the otherwise.