Originally Posted by SaiyajinPerfection

Even though he doesn't directly state it, I also believe that Vegeta reached Super Saiyan 2 during the seven year gap. Super Saiyan 2 doesn't become available to a powerful Super Saiyan who simply revs their ki, it comes from learning to rev it effectively. This is why Goten and Trunks weren't able to do reach it even though they also appear (however, are not confirmed) to be mastered Super Saiyans; their lack of skill and battle experience was why they couldn't ascend, and all the Super Saiyans with considerable battle experience and training could.

If a Saiyan was to simply rev their ki uncontrollably, it would result in SSj 3rd Grade, and not the more powerful and more effective SSj2. Trunks strongly implies this when he sees Gohan transform into a SSj2. I will now explain why Super Saiyan 3rd grade is actually closely related to the Super Saiyan 2:
With mastered Super Saiyan, it is not an alternate transformation of the Super Saiyan, it is the same thing.... but it has been trained to a point where there are no flaws, like excess ki consumption from maintaining the transformation itself and the hyped emotions. Therefore while Super Saiyan 3rd grade actually produces more ki then Mastered, Mastered is better to use and appears stronger because a high percentage of ki produced by the SSj 3rd grd goes to maintaining the transformation itself.

As for your original question about Majin Vegeta and why he didn't have bolts in his aura until he powered up against Goku, because he was only mastered Super Saiyan at that time. He hadn't transformed into Super Saiyan 2. This actually also supports my logic that SSj2 requires a conscious control in revving the ki, because the Majin possession is said to maximize your ki. If a maximum ki boost is said to also transform a Super Saiyan into Super Saiyan 2, then Vegeta would've been Super Saiyan 2 upon his posession, and not when he decided to transform against Goku.