Hmmm. I naively followed the vortekz recommendations for placing the VGs at decreasing angles starting from the vehicle center line on my own Jetta. I think there is some logic to placing the delta wing at an angle to the prevailing air flow. If you look at the "airtabs" style VGs, they force air to either funnel down through the rear opening or spill up over the top angled edge. I think the spill over may help to enhance the VG effect. It would certainly enhance local turbulence, which is essentially the purpose of the VG, to disrupt and trip the boundary layer.
Unfortunately it probably also adds drag, so I might reposition mine. I have seen people claim good results with the VGs parallel to flow. so it might not be necessary. It seems clear that the folks over at vortekz know absolutely nothing about aerodynamics though...
Current position:
