The pick up shots done in Tunisia were done during post production on Clones i believe so they would not have to go back again.
As for where it was shot i thought a good deal if not all was on set in a warehouse that used to be a rolls royce factory and they used green screen to shoot Vader's masking, Vaders NOOO and Frankenstein act, Vader the emperor and tarkin overlooking the death star.
The making of book really try to get into a sense of where Lucas wanted to take the direction of the film, a lot of ideas were not used many for the better. But changing Vader's turn from being seduced by power to saving padme from dying was moronic.