Vader was fine in Jedi. His change started at the end of Empire Strikes Back. His final shot, on the bridge of the Executor, when the Falcon escapes. His son escapes, and he stares longingly after him into the depths of space. He starts to turn, but then looks back, one more time, as in disbelief. When he walks off the bridge, the officers, particularly Piett, are fearful for their lives. And yet, he just quietly walks off the bridge.
No, Vader’s moment of vulnerability was just quietly walking off the bridge, back to the camera. Off to console himself with his own emotions and thoughts in some private chamber somewhere else on the ship.
Well sorry to go a bit off-topic but I have relate this to “Alert my Star Destroyer to prepare for my arrival.” thread in Theatrical Cuts vs. Special Editions section. That is the exact reason why I hate “BRING MY SHUTTLE!” line. It does not fit at all.
Back on topic, I think Vader is one of the high points of ROTJ.