Not to forget that Vegeta was dead. In DBZ dead people are supposedly stronger and more enduring than the living - that's why SSJ3 Goku's power runs out far quicker than he expected, but also why Vegeta managed to keep Kid Buu distracted for so long.
Basically, SSJ or SSJ2, Vegeta didn't have a chance against Buu. In fact, there was an entire episode dedicated to the fact that for the first time ever, Vegeta acknowledged that he was outclassed and that the only person left to save the universe was Goku. He'd never done this before, even though there were plenty of times somebody transformed and got incredible power while he was watching.
As Nite said, Vegeta tries very hard not to be impressed by power. When Piccolo fuses with Kami and is powering up on the ground, he's initially shocked in the air - but he doesn't let anybody see that, and instead confronts him angrily and demands to know how his power increased.
There's also the matter of him truly believing that he was more powerful than Trunks after the Time Chamber training, when Trunk's raw power was incredibly high. He just lacked the training and finesse to correctly use SSJ2 instead of USSJ.
Vegeta surely would have sensed Trunks' greater power if he wasn't so blinded with his own ego and new achievements.
He also refused to become Vegetto initially thinking that he actually had a chance against Super Buu, when it was obvious that none of them did. After all, if it were possible for one single person to beat Super (Gohan) Buu, Goku would have turned SSJ3 and done it himself.
So it IS possible that he was only SSJ against Kid Buu, if his Majin powerboost was truly gone. He survived as long as he did by being dead already, although he came close to being annihilated several times. The fact that Goku was SSJ2 and managing to keep up with Buu suggests that either Vegeta was SSJ1, or Goku was simply more powerful and skilled than Vegeta in any form.