The idea of the VGs is to create a swirl of air that ends up behind the vehicle reducing the low pressure behind it thus reducing the drag. So for your side VGs you need to look at the resulting swirl and add more if it doesn't do the job. I think that double fins on each like the airtabs may also increase their effect ... The idea is to kick the air up away from the body laminar air flow as smoothly as possible and have it return behind the vehicle. Having a Scion xB really has me thinking about installing some but the return on investment is questionable and the potential winter conditions here may damage them increasing the cost. I may try to 3D print some if it could create the proper shape with the cad drawing programs I have.
I'm glad I didn't pay money for them. Yes the AirTabs will probably be better but not convinced about these used as a MOD to solely give you better FE. Mine hasn't changed but the extra stability at speeds above 60mph has been great. More stable the faster you go, well up to 75mph is all I got to here.
The problem I have with the side VGs is the turbulence created by the large side mirrors and snorkel on right side. The air is turbulent all the way to the rear so the VGs on the D pillar are doing nothing really.
Anyway, for stability they work well. For FE, not convinced.