I like vent windows and wish they would come back. I think they'd be perfect on retro-inspired cars like the newer Challenger, Camaro, etc. But I can't see them saving fuel--quite the opposite. You have one of these open and you're increasing drag.
Mythbusters actually did a show (not with wing windows) where they tested how fuel efficient a car was with windows up, A/C on, windows down, A/C off, and windows up, A/C off. Not too surprisingly, windows up, A/C off was most fuel efficient, if least tolerable, A/C on, was next most F.E., and windows down used the most fuel.
You might argue that if you just had a floor vent you could open, that you could leave the windows up, but the incoming air has to come from somewhere, and for flow you still need a window down a little. While the fresh air intake on your HVAC system also is open to the outside, the fan eliminates the need for this to be an open 'scoop' wrecking the aerodynamics of the car.