[MetroMPG]

A pair of counter-rotating vortices are present at the rear of many vehicles. They were probably there before you added the vortex generators.

And yes, they're a "detriment to drag".

The way to avoid them is through shape optimization. Ideally you'd have a Kamm shape to your car (ie. extend & taper the greenhouse to the end of the vehicle) to get rid of them.