Vendetta

It's very interesting because I found a new way that only worked on my game but you can try it.

I unlocked Vegeta the same way I unlocked Teenage Trunks which was in the Hyperbolic Time Chamber (The second time I think it was) you go to the jar in the bottom left hand corner and get the lid to pop off by itself, then when it is open, keep pushing A on the open jar, after a few times you should get a new card, after you have got the new card, just keep going through the game as normal and once you get right up to the end of the game after you beat Kid Buu it will tell you that you've unlocked Teenage Trunks and Vegeta