MrInstrument - Aside from your results, do you happen to have any sources of study about using VG's on passenger vehicles the way you describe? You're describing a sort of "virtual tail" that I think would be very popular if it were doable.
My own A-B-A testing of air tab style VG's (which supposedly take into account the required angle to the local airflow to generate vortices) showed
no effect when placed at the rear of a minivan on the sides & top.
Yet a simple
Kammback style tapered extension did measurably reduce drag on the same minivan.
I'm open to the suggestion that I did it wrong, or that the VG's weren't optimal for the conditions.