I have read that hackers have hacked the disc and they said that vader and yoda exist on both versions
That's the idea. *Action Replay and hacked firmware cheat injectors would only allow someone to open up access to something that's already on the disk. *Vader and Yoda are probably on both disks, but they won't let you use them both because they made them exclusive.
Soul Blade (aka Soul Edge) and Soul Calibur 1 did not have guest characters. *Soul Calibur 2 had three guest characters, Link, Heihatchi, and Spawn, each on a different system. *Soul Calibur 3 had 1 "guest" character, Kos-Mos (you could only create her by using all of the "type-x" parts in create a soul - for this reason many do not consider her a guest character, especially because the announcer doesn't say her name and she doesn't have any profile details). *Soul Calibur Legends had Lloyd as a guest character. *Soul Calibur 4 of course had Darth Vader and Yoda.
This puts the games with guests at 3, and the games without guests at 3. *Guest characters hardly make the series "unique".