Do you mean the Flow Simulator
here's the N Beetle with a roof spoiler, it looks to me like the air stream is already in a good flow with out the VG's
The VG's may as they are designed, bring the air back down, in my case onto the hatch glass.
I cannot find the video of the beetle without the roof spoiler but it was around at one time.
A benefit could be found for other vehicles but for mine I fear it will cause lift-drag, I can now deduce, after revisiting this video.
or this next one, it looks more realistic