Morgan, You DO NOT want to keep airflow attached. VG’s are used to try and keep flow attached when a rear shape tries to change too quickly for it to remain attached but not quickly enough for it to be a clean break. VG's would be useless here, the slope of the Bugs back is such that flow is remaining attached way too long already.
Understand that when I say you are generating too much lift, it is not all in the straight up direction, a very good portion of the lift is in the backwards direction, which is a huge drag load on your car. This is why you want it to separate at the top of the windshield. If you fear that you are creating a huge area of dead air that is starting at the top of your windshield, you should not worry about it, because this “Dead Air” drag will be there no matter what you do to your car....I hope that makes sense.
Below is a basic illustration of what I’m trying to say. By keeping the air from moving down the rear window, you eliminate the large lift, (Black Arrow) and it’s associated Drag Vector (Red Arrow).
The "Dead Air" drag is about the same as "Aerodynamically Shaped Perfect" drag. As soon as the air on your bug starts going down past the top of the rear window, it is costing you. So VG’s would do no good since the flow is already attached.
Go with a roof spoiler as I suggested earlier, it will yield the best results.
The dead air that forms will be less drag than the lift.