If you imitate Darth Vader, I don't think James Earl Jones has as much case for likeliness as Star Wars franchise
If you just want ScarJo's (or James Earl Jones') voice, you need the rights from them. Period.
If you want to reuse the character of her AI bot from the movie (her name, overall personality, tone, rhythm, catchphrases, etc.), or the character of Darth Vader, you also need to license that from the producers.
And also from ScarJo/Jones if you want the same voice to accompany the character. (Unless they've sold all rights for future re-use to the producers, which won't usually be the case, because they want to be paid for sequels.)