thanks guys!
The way the VG's are mounted on the wings are exactly as Jon mentioned..
The VG's really take effect at HIGH angles of attack.. (ie right before the wing stalls)
The aircraft stalls because the aircraft exceeds the angle of attack and the airflow no longer accelerates over the leading edge of the wing and buffets behind the wing.
If you notice the way the VG's are mounted on the Cessna's, at the highest angle of attack to maintain slow flight (55kts) the VG's are just below the highest point of the leading edge.. allowing the VG's to take effect and keep the air flowing and accelerating down the wing.
Now in a Z car.. the air flowing over the top of the car actually accelerates down the rear hatch before it seperates from the boundary layer. The point of the VG's is to increase the boundary layer seperation and keep the air accelerating down the hatch, which increases the effectiveness of the spoiler!