Winglets reduce the effect of span wise flow, which increases the strength of wingtip vortices. Increased vortex = increased drag (vortex drag which is part of the induced drag ie drag which is induced by the aircraft's velocity. Not to be confused with form drag etc which is a zero-lift drag function. That said, winglets have a surface area and therefore produce some form drag but I digress....)
They certainly reduce fuel consumption in steady state flight (the cruise) because like for like an aircraft with winglets will have a lower total drag and therefore require a lower thrust setting for a given speed or will be able to cruise faster for less fuel if you like.
With regards to 787, 747-8 etc with so called raked wing tips is more to do with aspect ratio but as you will note have a large degree of sweep relative to the rest of the wing at the tip. This serves to minimize the effect of the dastardly vortex. Aspect ratio being a function of span2/area is enhanced by increasing the span. Aerodynamically perfect wings would be infinitely long (amongst other things) to prevent the wingtip vortex ever forming.
This would of course make it a sod to park...
In answer to the post. Winglets will affect vref, but only slightly. A wing with winglets will stall at a slightly different speed to one without but not more than by a couple of knots. Both lift and drag are directly proportional to surface area and winglets obviously increase both. Like anything, hopefully the benefits outweigh the negatives.