Vortex generators belong to a set of "bandaids" used to produce a turbulent boundry layer. A TBL is thin and quite "sticky" so it aids in delaying the formation of an upper surface separation bubble and eventual stall.

The thing is that this is not an automatic sort of thing. If you pick the right airfoil for a model then the airfoil itself can produce the same results without requireing that you resort to these fixes.

The tubulation also needs to take place ahead of where the air generally separates from the airfoil as the angle of attack climbs. If it behind that point then it won't do any good as it's inside the turbulence of the bubble.

Possible options for forming this turbulence are numerous. The little vertical angled bits are one option. Another is a simple raised strip such as you'd get from using two or three layers of 1/8 inch wide trim stripe tape added over each other to build up enough thickness to be suitable for the size of the airfoil chord. Others are cutting a saw tooth patthern in thick tape and adding it to the airfoil.

But all the results are the same. The early laminar flow is churned into a thin TBL in order to delay separation and the final stall event.

What works and where it should be located all depends not only on the airfoil used but the building method, chord dimensions and wing loading. All these things have an effect on where a possible separation can take place. But generally it's a safe bet to locate a trip trip, or other boundry layer trip at around 15% of the chord.