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My understanding of VG's is they are used to, as mentioned earlier I think, to create controlled turbulance which achieves a thicker boundary layer that promotes clear seperation before the bend where the air would normally wrap around the vehicle creating major vortexes & negative pressure zones, in doing this they are supposed to simulate the effect of a short kamback/boattail which produces a smaller wake area that is set back from the vehicle itself. Like on the Mitsu, by disturbing the air at a higher point before the rear window they effectively soften the angle and allow flow to remain relatively attached, albeit with a thicker boundary layer to the wing on the trunk. So theoretically they can be used to maintain attached flow on angles that are just a little too steep or reduce drag on squared off vehicles, when used on the underside the idea is to use this thicker boundary layer to simulate a belly pan.
From what I have read they do work, but it requires a lot of testing on a specific vehicle to get the right placement and spacing, hence the ratio of failure to sucess is 10:1 or more.
I imagine a simple 2-4" plastic strip taped flush to the body (like mini boattail) following the template lines would achieve greater success as it would promote clear seperation, reduce wake size and push wake further away without the increased frontal area and the ugly factor wouldn't be any worse.
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