dirty sanchez
Winglets reduce the effect of span wise flow, which increases the strength of wingtip vortices. Increased vortex = increased drag (vortex drag which is part of the induced drag ie drag which is induced by the aircraft's velocity.
This statement is completely backwards...
and there is no such this as wingtip vortices.
The discontinuity of viscous flow over the top of the wing is the cause of drag....winglets tend to move the area of discontinuity outward along the wingspan, hence less drag ...it likely has more to do with winglet/wing bending, hence the newer scimitar and active winglet designs...nothing to do with the vortices that the wing creates.
In regards to advanced wing design...one should note that even the 787 has vortex tabs down the wing...yet Airbus products do not.