Lift (or downforce) works because of low pressure on the suction side of the wing (or in this case diffuser). The basic way this low pressure is created is by the wing pulling away from the airflow. Consider a diffuser -- it pulls away from the ground, expanding the airspace. The same amount of air in more space equals lower pressure.

However, the air will only follow the surface of the wing so far. When the energy in the boundary layer gets low enough, the air will separate from the wing. This drastically reduces lift and drastically increase drag. On an airplane, we say that the wing has stalled.

A vortex generator adds energy to the bounfary layer. But this energy does not come for free -- it costs in drag.

So IF the wing is stalling, and IF the vortex generator is placed correctly, then SOMETIMES it can keep the wing from stalling and decrease drag (or otherwise improve the airflow).