hapter: 319 (DBZ 125), P9.4-5, P10.4-7
Goku: “I’m a Saiyan who came from Earth to defeat you…A legendary warrior whose tranquil heart has been awakened through intense rage…Super Saiyan Son Goku!”
Freeza: “I…I knew it…It seems you really are a Super Saiyan…Fu-fuffuffuh…So your tranquil heart was awakened through intense rage, huh?...I see, so that’s why Vegeta could never become one, no matter how hard he tried…”
It's always been indicated that a tranquil heart was required for activating Super Saiyan. Practically all the SSJ's we've seen so far in canon are far and away from the bloodthirsty savages typical of their species. Vegeta wasn't exactly goody two shoes but I'm pretty sure it was mentioned somewhere that earth's environment allows for an increase of S-Cells, plus he did say he had a calm heart... albeit pure evil.