Very interesting post.
This would require a look into some fluid dynamics for sure as it has been pointed out.
Flaps would indeed seem to promote the separation of the vortex near the flap tip, where it had lesser opportunity to gain spanwise flow strength. Again I ain't no expert either and i am just speculating.
And here is another question, why does the 757 have the strongest wing vortices? Does it have anything to do with a large wing proportionate to the fuselage which is quite skinny or am i on the wrong track.
cheers