Because of the nature of the film its difficult to turn Vader into the ESB version we'd like him to be more in ANH. He simply wasn't written that way back then and the whole saga was basically just vague notes.

However fortunately "difficult" doesn't mean impossible...in fact it makes things interesting. : )

This is why I'm loving the suggestion thus far about giving him a genuine moment of his own to "meditate" and sense Obi-Wan as opposed to that moment just being a matter of fact sort of thing.

Also Jai's vids showing Vader moving slower and my mp3 with the lowering of his voice are all small things that can make a huge difference.