JL,
Winglets (or wingtip sails) do not actually reduce the strength of the trailing vorticies. The vortex strength related to the wing lifting vortex, which in turn is related to the lift generated by the wing. There is nothing that can be done to reduce the trailing vortex strength - bar changing the shape of the wing planform.
Winglets take advantage of the local flow induced at the wingtips by the trailing vortex and use it to both create extra lift and create thrust (believe it or not!). This is how the extra efficiency is achieved. Further, they operate best in steady flow conditions, and thus, are best installed onto aircraft that cruise for long periods of time. For short range aircraft, I can't really see a point in installing them.
Although I have no doubt that they do work to a degree, I'm inclined to agree with tagaroo in that they are probably there for show as much as function - especially when you consider the fact that SOP allows individual sails to be removed. However, I am not an airline pilot, and I may very well be totally wrong!
Cheers
Cuban_8