THIS.
I saw a making of featurette on Halloween and Debra Hill was gonna double for Michael Myers for some distant shots and they just scrapped it because her posture and walk was totally different to Nick Castle's. It's very apparent in Halloween 2 that it's a different actor playing the character. Also you can bring ROTS but Hayden didn't do much more than being strapped to a table and standing. Even ANH Vader walks very different to ESB and ROTJ Vader. The only scene in ANH where I see the Vader from the other two is when he goes down to the hangar to check the Falcon.
I don't pick up on Stormtrooper stuf, but the Vader suit didn't look atrocious to me. I recognized it as ANH-style, I mean there's differences between scenes in the same movie, there are 3 visibly different helmets in ESB, then there's the chest armor and the shoulder bells changing position between scenes and he just simply goes matt from shiny in ROTJ.