In an interview with the Xbox Community Network, the Director who worked on Soul Calibur IV (Katsutoshi Sasaki) mentioned that the team had no plans for bringing Vader to the Xbox 360 version and Yoda to the PS3. So there you have it, although things could change in the future if both camps shout in their loudest voices at him.
Update: Maxiconsole has posted a video which shows Vader and Yoda fighting on the Xbox 360 version, even unlocking an achievement. Does this mean Vader is on the disc? Is the Director telling porkies?