Thanks for your responce.
One negative point about the insallation of VG's , I was told, is that once you "tamper" with the fusselage one could have problems to keep a plane in a life extension maintenace program.
Whether this is true or not I don't know I'm investigating this.
I also was told that for a C172 stall speed would only go down by a few knots but that aileron responce on slow speeds does improve a lot.
The same for the elevator, but this is not always an advantage, because due to the increased response one would be able to pull the plane into a deeper stall instead of the easy to recover stall the C172 usually has.
Anyway I'm still looking for info on it if I find more I will post it.
Greets