If you mean area under the car, then there isnt much chances for the flow separation to occure. The tarmat keeps the flow connected to the car body anyway.
Vortex generators make sense only if the alternative is a flow separation. if flow is attached, vortex generator will increase the drag.
So, you put vortex generators where they are absolutely necessary, where there is enough volume for the flow to separate, and sufficiently large path has been passed by the air to accumulate turbulences that leads to flow separation.
Usually this means at the end of the car's bottom, as show in image, only there flow separation can occure. For top parts it could be at their middle part, as there is much more volume around to accumulate turbulences. And in case of large turns for air it might make sense to put vortex generators in the first third of the part, as flow separation becomes more likely even with little accumulated turbulences.
There is a way to calculate it. It usually includes distance from the start - to estimate how much turbulences each part of air has accumulated so far. And what is the hydraulic diameter - flow gets streamlined in narrow channels by itself. With significant curvature I didnt see a way to calculate it, and simulators are usually used.