The difference is that you don't need to do it in a Tyler Durden way...
The EditDroid line of "Obi-Wan would have told you long ago, had I let him" now eliminates this need to save Ben's diginity. Ultimately, it means that Yoda orchestrated the whole thing as a plot to bring down the Empire - what better way to do this than to instill a desire for vengeance in Luke, carefully nurtured to make sure he doesn't stray to the dark side. He is the last hope (pretty much) and desperate times call for desperate measures.
With this in mind, new completely separate footage could be filmed and worked in to RotS of Darth Vader moving through the city, doing messed up things. In at least one scene, make sure he refers to himself simply as "Vader". Make sure it only happens at night. And as soon as you disassociate Darth Vader from Anakin, add a few subtle hints in to make a very astute first time viewer wonder if maybe Anakin has split personalities, that "Vader" comes out at night. Maybe a rip in both of their robes or something small but noticeable when you look. Add bags under Anakin's eyes in daytime scenes directly following night time activities. Find a way to work "Vader" into a conversation, and perhaps put up wanted posters.
But if nothing else... if "Darth" is going to be used as a Sith title, I think it is important that if Vader shows up at all prior to Anakin's "death", that he always be referred to as "Vader", never as "Darth Vader". He only gains the "Darth" title after Anakin's "death", when the second personality takes over.
