ludovico - The key thing you need to look into is
Induced Drag.
This is the drag created as a by-product of lift when the angle of attack (AoA) increases towards it's maximum.
I think you may be getting confused with where the vortices are coming from... To quote wikipedia... (somebody had to!) The strongest vortices are produced by heavy aircraft, flying slowly,
with wing flaps extended.
So while that example gives the strongest vortices, they aren't occurring at the wingtip. Even stronger vortices are created by the flaps with the slow/heavy conditions mentioned, and as previous posters have commented, this happens on the edge of the flaps, not the wingtip.
So the statement as far as WINGTIP vortices are concerned is correct.
Hope that makes some sense...
12/9/08 - And to think I actually believed wikipedia was right..! Ignore the part about having the flaps extended.
I'll stick to little planes in future...
So does this mean that there would be greater wake turbulence encountered behind a clean airliner (heavy, low airspeed & @ high Alpha)...? Or is this a different matter altogether??