So these wings have stronger tip vortices to work with
It is a common misconception that the wingtips themselves create the vorticies. Winglets and sharklets do nothing to the creation of the vortex.
The vortex is a rollup combination from the bottom surface of the wing structure, (for lack of a better example, a boat plane in the water) coupled with the rollup from the top surface of the wing.
Just as a boat planing through the water, the surface area in contact, the shape of that surface, and the directional flow as a resultant of that surface, are what create the vortex.
The winglet or sharklet, tend to reduce the drag associated with the outer portion of the wing, where the camber and length are at their greatest discontinuity, winglets/sharklets reducing the turbulent airflow in the last portion of the wing, ie cavitation, thus reducing drag.
Winglets/Sharklets, in reducing the cavitation, in smoothing the airflow, will tend to optimize the particular wing for its particular vortex creation. This is not necessarily a bad thing, and each configuration must be looked at distinctly.
Just as the angle of attack differs with conditions and aircraft loading, so does the resultant vortex. (Leave out the weight, focus on the aircraft config at load)
Current designs of winglets/sharklets rely on a certain configuration...
this is similar to fixing flaps at a certain configuration... hardly optimized flight.