Actually, the voice of Vader was dubbed. I believe the actor playing Darth Vader was Dave Prowse, while the voice was supplied by James Earl Jones. TM’s voice was incredibly distinctive and powerful, and would have done very well.
Mark
Mifone was always knocked for his voice - it is very gruff and rough. People came to imitate it thinking that all Japanese actors sounded like that. I also like Red Beard - if you can find the scene where he takes on about 5 or 6 ruffians, it’s well worth watching.
I know Vader was dubbed; I’ve even seen the test footage where they tried to have the actor inside the suit actually read the lines, and it completely ruins the effect. Granted that TM could have read the lines to be dubbed in, but his real strength is his stage presence, and that would have gotten lost if they had just had him doing voice-over. His voice is more gruff and slightly abrupt, and without much strength behind it, which I think would have lessened Vader’s impact on the screen. In the end, a voice like James Earl Jones, or Brock Peters (who did the voice for the Public Radio drama) allows for a better impact to the character; without a face to emote with, everything has to come through in the voice, and TM’s voice just doesn’t convey “7-foot hulking black-armored giant”.