Ceppo,
I'm afraid that there is not much one can do to reduce the strength of trialing vorticies behind an aircraft - and you certianly cannot do it through the use of winglets! Winglets use the vortex pattern shed off the wing to increase airframe efficiency. Further, the downwash angle induced by the aircraft is directly related to the lift produced by the wing - reduce the downwash, reduce the lift produced!
My own thinking with regards to the 777 situation is that due to the increased wing efficiency over that of the 747 or 767, the company has calculated that the net benefit of winglets would be marginal or negetive.
I have been meaning to ask my departments Prof. about this for a while - he's an international authorty on wing design and might be able to sort out our problems!!!
Regards,
Cuban_8