Spaceman, Remember back to a graph of drag versus airspeed where it shows curves for induced drag and parasite drag. This is the one that most people use to show total drag and L/DMAX
(which would of course be the lowest point on the DI
curve. I don't have any reference books nearby at the moment, but this graph should be in every private pilot manual ever produced.
Remember that the parasite drag curve starts low and increases with airspeed, while the induced drag starts high and decreases with airspeed. If the Canadians don't teach this on the PPL, you may have to look this up, but it is commonly held knowledge that wingtip vortices are the greatest when the aircraft is heavy, clean, and slow. That is because wingtip vortices are part of induced drag and become weaker as airspeed increases (other factors being held constant).
It can be explained by other methods, but many people can grasp the non-technical explanation more readily.
On another note take a look at this:
Notice that looking at the rear view, the upwash fields extend well beyond the span. "Wingtip vortices" as we call them is a bit of a misnomer. What most people teach regarding the flow leads the student to believe that it is only the flow in close proximity to the wingtip that causes wingtip vortices. The downwash created by the wings is part of this same complex flow pattern. We break it down into many parts to make things simple to understand, but it all fits together.
For this reason, I understand why you believe parasite drag plays a factor. To change your mind, you may have to lead yourself to realize that the flow patterns which induce "wingtip vortices" are merely centered about the wingtip, each flow pattern inducing the vortex have a radius of at least one half-span. I suppose you just have to look at things on a broader scale with an open mind.
I wish I had graphics to show everything a little more clearly, maybe I will try to post others at a later time.