I would think that a blade inside a duct wouldn't allow the formation of vorticies at all. Thats probably why you never hear about engine manufacturers (like GE or RR) worrying about vorticies. The only problem with a duct is that you cannot get the thrust that a large diameter, non-ducted blade can deliver.

Winglets technically only reduce the effect of voticies, not eliminate them. So, like airraptor said, its like adding wingspan with the same sized wing (for airliners that is). I don't know about props. I would like to see scientific data on this one before I start talking about the effect of winglets on props.