My feeling about the Vader suits is that a good case could have been made for Vader updating his suit between films if the suit in ROTS had looked quite a bit more retro than the one he wore in ANH. (see the Radical Prequel Redux Thread for details).
Instead we got a very tidy hybrid of all the OT suits which looked more advanced than the one he wore in ANH, which like so many of George's decisions creates more problems than were there before hand.
If someone fixed the ROTS suit the updating theory would automatically correct any continuity issues with Vader in the OT.