Is there any indication that Vegeta can get to SSJ2 at will?

[Bussani]

I don't see someone getting weaker by not training. I think it's more or less that they get rusty. I think that GT Goten and Trunks would lose to their younger selves. We all know pre-Elder Kai Gohan would lose to Cell Games Gohan... but I think, not because of some sort of power gap, but because of the mental and physical shape that he's in.

Vegeta became frustrated when Gohan was unable to defeat Dabra, claiming that he was stronger when he was a kid. This implies that Gohan at the Cell Games was stronger than his Boo Arc incarnation, and that he would've been able to defeat Dabra. This is Dragon World, not real world. Anyway, Gohan had become less rusty after training with Goten for the Tenka'ichi Budokai, and he certainly didn't appear rusty when he got his Mystic power-up and began pummelling Super Boo. That was to do with power, which basically eliminates any rustiness Gohan had gained in those 7 years of peace.

After Dragon Ball (pre-Z), power's the most important thing you need in a fight. It is stated in the manga that Son Gohan decreased in power due to not training.

Ki isn't purely tangible power, though. Bringing it out and controlling it is more skill than physical strength. Even in the Cell games, Gohan couldn't willingly bring out his full power until the very end, with Goku's prompting. After 7 years of not using it and never even training to get full control over it (like Goku did with SSJ between Namek and his return to Earth), it really could be that he was 'rusty' mentally.

We know for a fact that Gohan has a ton of power that he can't consciously use. That is, until Elder Kaioshin's power up. That's why he's no longer 'rusty' after that.

In other words, power and skill go hand in hand when it comes to ki, and your raw power is affected by your mental state.

[Akira]

Kaboom cleared this up perfectly with his initial response. Others added legitimate points that clear the matter up further. One can sit and systematically forge a semantics argument on any point or issue in the entire series if they really want to.

There is evidence enough to prove Vegeta was already capable of Super Saiyan 2 before hand. No way he stood around for 7 years and let Gohan be the strongest on earth. It was evident early on in the Buu era that he had been training every day to surpass Gohan. Bulma even says that training in the gravity room is all he ever does.

Vegeta jumped at the chance to fight Gohan at the tournament and prove his superiority. Super Saiyan 2, as he witnessed it at the Cell games was his goal to surpass. He reached that goal, then rested in the fact that he was superior, but trained to maintain it.

He assumed he would be dominant at the tournament, and was even more excited at the chance to face Goku instead of Gohan. But the appearance of Kaioshin, Kibito, Yamu and Spopovitch, along with the interuption to his moment of glory that they caused, infuriated him. His chance to fight Goku again was looking less and less like a posibility.

Then, when he sensed for a moment, Goku's power at Super Saiyan 2, which was not only beyond what Gohan showed at the Cell games, but also well beyond his own Super Saiyan 2 power, he decided to allow Babidi to control him.

The decision was two fold, first to gain the power beyond his Super Saiyan 2 that would make him a match for Goku, and second, a way to force Goku's attention on fighting him.

That is the most logical explanation for the sloppy Buu era stuff relating to Vegeta and his power. The fight with Little Buu was, as mentioned, handled differently than in the original Manga. Anyway, looking at it any differently than this is because you choose to, not because you can make an argument that makes more sense than the obvious.

That is always how I interpreted it to be, even from VHS fansubs I originally watched of the saga. I've never understood thinking that the powerup caused his SS2 transformation, there is nothing hinting at, supporting or even directing one to think that.

I admit that explanations are handled poorly in that saga, but there is enough there to make sense of it if you pay attention.