I've got a downloaded clip from somewhere from footage of shooting of Ep III.
It was very bad quality but there was enough to make out Darth Vader.
He had no helmet during this scene and was just wearing all the black stuff seemingly just for the hell of it. Just to be cool and evil and all "Dark Sidey" and stuff. "It seems my plans have been interupted..." ,etc, etc....
He was hanging off of a cliff edge which I can only assume is into where he falls when he is fished out and placed into the mask, breathing apparatus and becomes the Vader we know from the first films. It is a lava pit and he falls into it fighting with Ewan Kenobi.
What concerns me more, however, is the possible titlename, which AKAIK has yet to be released. One is troubled by the possibility of something extraspecially cheezy like "THE DARK PLAN" or "THE REVENGE OF THE EMPEROR" etc, etc...