While I would love to see Vader in action, especially taking on multiple jedi, while he's in his prime. There might not be a lot of time to see him fight (as Vader), from the rumors alone about the duel, it will most likely be very long and probably toward the end. Unfortunately I think the end of the movie is when were gonna get our first glimpse of Vader, perhaps the move will end with Palpatine giving Vader instructions to eliminate the remaining Jedi. I think ending the movie with Palpatine talking to a shadowed figure, and the right before the credits we see Vader in his full glory step out, with James Earl Jones saying "As you wish my master" or something to that effect. Now thats a dark ending!