[If this devolves into crazy-talk, kick it to the corral, but we can at least start out trying to improve understanding of this topic.]
VGs can be discussed rationally, but claiming they'll do anything other than energize the boundary layer, and that the energized boundary layer will do what energized boundary layers do, gets off into "guess & hope" territory pretty quick.
VGs are thoroughly studied and well-understood devices, but that understanding is, while fairly solid, not particularly widespread. Their direct aerodynamic function is usually at least one order removed from the reason for their employment.
For instance: a row of vortex generators on the vertical stabilizer of an airliner reduces drag overall. However, the way in which it reduces drag is not direct. The VGs energize the boundary layer behind them on the vertical stabilizer. This allows the airflow to remain attached across the junction between the vertical stabilizer and the rudder (the movable part of the vertical fin) making the entire airfoil more effective (capable of producing more [horizontal] lift) at a given airspeed. Since the vertical fin assembly is more effective, it can be smaller, reducing friction drag and weight. It also means that it takes less deflection (angle of attack of the entire vertical fin assembly) to produce the same amount of horizontal lift, which reduces induced drag (by reducing the rearward lift vector). (The size/weight/friction reduction and reduction of rearward lift vector are factors that have to be balanced against each other and optimized.) Clear as mud?
Now, the reason all this matters to you as a frequent flyer is that rudder effectiveness is most important in a situation where one engine (on a typical two-engine airliner) is not producing thrust (broken, on fire, or has fallen off, for instance).
A more effective rudder allows more differential power to be applied without making the airplane uncontrollable (It's a factor in Vmc, for you pilot types). Since one engine is making no power, an increase in differential means an increase in total power. More available power lets you carry more weight in this situation, so the VGs on the tail allow the airplane to carry more weight in a given situation, which allows the operator to sell more tickets & make more money. (Operators are limited to carrying the amount of payload that would still allow the flight to be completed even if the most important engine fails at the worst possible time.)
So in this case the VGs reduce drag and allow the airplane to carry more weight. It's an indirect connection, though, and if the direct increase in parasitic drag of the VGs themselves isn't overcome by the other factors, they won't make sense, and won't be used.
Briefly mentioned here.
Applying this to vehicles, we have some different goals. Essentially, we're usually looking for direct drag reduction, not increased lift (or downforce). So the VGs will tend to (through energizing the boundary layer) help the airflow remain attached. If attached airflow will allow a reduction in drag, greater than the increase in drag of the VGs themselves, there will be a net decrease in drag. However, it is not at all clear that the reduction in form/pressure drag is likely to be greater than the increase in parasitic drag.
In situations where the airflow separates (relatively far forward), AND doesn't reattach, AND this is a major factor in increasing the (form/pressure) drag of the object, a row of VGs place forward of the separation plane MAY energize the boundary layer enough to allow the airflow to remain attached and reduce overall drag.
For experimenting, a rectangle of aluminum, bent in half crosswise and taped to the surface at an angle to the airflow will suffice to play with VG effects. There's no need to buy some specially-designed VG to see what effect energizing the boundary layer will have.
Nomex on, awaiting correction, happy to learn.
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