B-15 HOW CAN VEGETA BECOME A SUPER SAIYAJIN?
Referring back to question B-13, Vegeta satisfies both the first and second qualifications easily. As for the third and fourth rule:
After seeing Trunks and Goku in Super Saiyajin mode, Vegeta not only realizes the level of power of a Super Saiyajin, but also the fact that there can be more than one Super Saiyajin at one time. Vegeta's been trying to reach (and surpass) the power level of a Super Saiyajin ever since. However, while he was training in the artificial gravity room, he realized that he has reached his limit, yet is still signifigantly weaker than a Super Saiyajin. Vegeta became enraged with himself, which in turns triggers his change into a Super Saiyajin.