I re-watched the lightsaber duel because I was intrigued by the suggestion that Vader's vocal reactions were a technical error - that the "aagh" was originally meant for the steam and the "oomph" was meant for the fall, but they were accidentally switched in editing. The "oomph" sounds fine to me and I don't think it needs to be changed. Even if he tried to scream in pain or surprise, it would be muffled because of all the steam blowing into his face.

IMHO, the "aagh" during his fall off the platform just sounds like bad acting. This is probably why the Blu-Ray, Dark Jedi's V3, and Ady's Theatrical Reconstruction all use the same SFX for both the steam and the fall. They only sound different because the steam hose noise muffles the high frequencies in the first instance. I haven't fired up my "Faces" VHS tapes, but I'm guessing this is probably where the "aagh" was first replaced.

Since I'm already throwing gasoline on this fire, I'm also going to suggest that it's not necessary to remove the sight of Luke's head after he leaves the platform off of which Vader "falls". He jumps down to a lower level then stands up. Some have said before that he "bounces" because there is a trampoline there, but after watching multiple versions of the film, he clearly just stands after jumping down to a level about 5 feet below the platform.

Piranhas, attack!