[Piotrsko]

Vortex generators were originally engineered to disturb the airflow over portions of an aircraft wing so as to make it controllable during stall scenerios. In a stall condition, they do reduce drag somewhat, but at stall the airfoil is generally at max drag configuration. Spoilers do kinda the same thing and any drag change would be the result of serendipitous airflow improvements. With todays supercomputers, maybe not serendipitous any more. The back in the day useage case for spoilers was planting the tires more firmly onto the ground to corner faster which unsurprising made elapsed times faster. Didn't do much at drag strips for better ET's, but they didn't head for the fence as bad.