Soul Calibur IV director Katsutoshi Sasaki said in a recent interview, that's what. Specifically, Sasaki responded "we don't have any plans for that" when asked by XCN if we'd ever see both characters from a galaxy far, far away battling each other on the same console.
You can't get much more official than the game's director, but his comments don't explain screenshots (such as the one above) and video that have appeared on the net clearly (and seemingly authentically) showing both Yoda and Vader in the same version of SCIV. We can't help but think Sasaki is practicing his Jedi mind tricks on us with this one.
[Via PS3 Fanboy]