According to previous extreme-emotional states, that's all a Saiyan needed to trigger a transformation... albeit a specific level of power, although you can see with some fighters, like Vegeta, it was more difficult because of persona & behaviour (you would assume). I've also heard that Vegeta on Namek had enough power to reach SS but he lacked the potential, or the reason there of. Now, as we observe Vegeta later, he trained and tripled his strength, far surpassing his powerlevel on Namek. It only took one emotional outburst (in space) to trigger his transformation. If we look a little further in the Cell saga Vegeta trained for one full year inside the Time Chamber, which tripled his energy again, right? After the Cell Games Vegeta was said to have trained even more. Now, if he'd witnessed Gohan do it, and he was smart enough to understand Goku's training method inside the Time Chamber, (though he never followed through with the method), he reached SS over 3 years of extreme training, and ascended to (Stage 2) of Super Saiyan ("Super Vegeta" - "USS") within one year, then I'm sure after 7 years of training -in the gravity room- he'd manage to figure out how to ascend to SS2, and then eventually understand how to do it at will, similar to the way he gained full control over SS. Not to mention; If Gohan was able to move up to SS by the mere thought of his inabilities & witnessing his friends being harmed then this should also apply to Vegeta, and it did. Watch as he reached SS. The only thing about SS2 I see as a problem for Vegeta was it required more of an emotional upheave than SS1, which conflicted with Vegeta's nature--having difficulty finding a pure reason to ascend - but with 7 years of training under a supposed "500x gravity setting" you'd just have to figure Vegeta was able to reach the form in a similar manner Goku did. You also have to understand Gohan's level of strength at the time of the Cell Games - he also had a very solid reason for moving up to SS2; the witness of injury & death. It's something Goku and most of the good guys couldn't bear to watch. This is usually why it was easier for the good Saiyans to transform, there was a necessary time--in Vegeta's case, it was more about desire and hunger for power, therefore it never played into Vegeta's benefit.
EDIT (Slightly off-topic):The idea Goku reached SS3 and practically used the form at will is beyond my comprehension. This is why I find him far too overrated for his own good. Goku, in my opinion, only shined during the time he surprised people with his Kaioken, his impressive & relentless attitude and will to stop evil--his first transformation was unique and genuine, which I think is what made Goku stand out as the hero. I prefer Vegeta though; I believe every bit of power he earned was well-deserved, his achievements deserved recognition & so did his changes--over the duration of the series he reveals to people his softer and more compassionate side, which is what I think made Vegeta stand out far more than the rest.
When we're brining up all of this discussion about Super Saiyan levels and whatnot it makes you wonder how far Toriyama actually went. SS2, alone, was fairly exaggerated next to the original form of SS, and so it only opened new paths for Toriyama's restless mind. I mean, you had to have the fusion of the kids, the SS3, the Vegito, the exaggerated and unbelievable levels of power, etc, etc. And the big and dumb controversy that followed in its wake; "Who's stronger than who" crap, "What attacks are the strongest?!?", "WAS TRUNKS SS2 WHEN HE FOUGHT CELL!!?!?", "isn't this form and that form SS2?!?!", and the amazingly annoying kids who yell and scream such mindless crap like "Goku's the strongest!!!!!!!!!! VEGETA SUX, BLAHBLAH"--the fact the franchise has spawned so much controvsery worldwide is quite a feat, considering the percentage of people who like Dragon Ball Z is practically the same as the percentage of those who dislike it. That tells you a lot about how much of an impact this one man's ideas have made. This thread alone is a good example of it.
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