They probably just got Peter Serafinowicz (who also voiced Darth Maul) to do his Vader impression. Whoever did the voice, it definitely sounds off and is the most jarring scene in the entire movie because as others have noted, it's just .... all bad.
I'm waiting for this subplot to be re-instated in ANH and get written up as canon for the EU:
http://www.youtube.com/watch?v=EtMkXEO2vs8