Vortex generators can be used to improve the performance of aircraft in a variety of ways. If properly sized and positioned vortex generators can be used to lower the stall speed of an aircraft, improve stability and control during maneuvering, decrease turning radius, decrease takeoff distance and increase takeoff weight. For commercial aircraft this can mean higher payloads and increased safety. While the general aviation pilot will enjoy better maneuverability as well as the increase in safety.
Straight from this site:
Aeronautical Testing Service, Inc. Vortex Generators explained.
They do not increase speed or save fuel. Added lift and stability at the cost of aero.
Thats the simple version for airplanes.
For Vehicles. Vortex generator work like this:
You increase drag at the point of vortex generation. You create turbulent air to flow past a trouble spot such as protruding mirror only to retattach as laminar flow. Thus actually creating less drag than the vortex generator actually caused.
I see these on the trailing edges of the roofs on some vehicles such as the Lancer. You need computer simultation or wind tunnel testing to properly place these. I would not try it because you would probably do more harm than good unless you have access to proper testing.