He was Vader for the latter part of ROTS, and he was nothing like the OT Vader. The same corny dialogue. I read something a long time ago someone wrote about an exchange between Vader and one of the folks he killed on Mustafar. As soon as Vader entered the room, one guy came up to greet him and said, "Lord Vader, let me be the first to..." And Vader says, "Very well, you will be the first." And then he quickly slices him with his saber and then kills the rest of them. Something like that would have been a lot more Vaderesque, in my opinion. His cold, witty, articulate way of expression is one of his defining features as a character - in the OT, at least.