No evidence it has heat of that caliber.
Doesn't need to be, Ki energy has been shown to vaporize beings that are lower in power, and has only ever been destroyed by Ki attacks of similar power. Gaara's sand will just disintegrate when it comes into contact
He's faster than Vegeta and he can fly.
Again, faster.
Not naturally and instantly like Vegeta can, and Vegeta becomes faster as an Oozaru. Its not like Gaara knows all his techniques, so he won't instantly think go for the ball, though he can still deduct it. Though by that point Vegeta will be an Oozaru that is not only much more powerful and faster, but will also not give Gaara the change to destroy or seal it.
- On that note, the Sealing would require the powerball to be dragged down to earth so that it could be sealed propery (as I am not sure if the sealing can be done in the air), but the ball is (again) energy so it would be impossible for the sand to actually "Grab" it, or even touch it.
Sealing as in just encasing it in a ball
Gaara's sealing does not work that way. His sealing requires the seal to occur close to the ground and cannot be done so in the air. Not to mention he has only ever used it against physical beings an not GIANT BALLS OF ENERGY.
but the ball is (again) energy so it would be impossible for the sand to actually "Grab" it, or even touch it.
He just has to block it off
The oozaru form doesn't necassarily need to have direct exposure to the moon or powerball once the transformation occurs, otherwise they would just transform back whenever they look away from the moon. Once a transformation occurs, they just need to be simply exposed to the Blutz waves to be transformed still, and Blutz waves is essentially just radiation, meaning that the blutz waves would still get past the sand and keep vegeta transformed. Only by destroying the planetary body or powerball can the Oozaru transformation be undone, as well as by destroying the tail.
- Finally, while not a surefire factor, Vegeta could also just use the ACTUAL moon to transform. Granted, its not a full guarantee that this will happen in the fight due to lunar cycles and such, but in the manga Vegeta DID plan to attack Earth during a full moon to take advantage of the Oozaru form (unaware at the time that it was destroyed a year ago), so I could see the same happening here.
Gaara can block it, or seal Vegeta.
... I've mentioned the part about the blutz waves earlier, even if it wasn't true, are you SOMEHOW suggesting that Gaara can block the ******* MOON, when his range is only a few kilometers at most and probably doesn't have enough sand to cover it?!
...I have nothing to say tbh other than you're a moron. Also if Vegeta did transform with the moon, it would probably be even harder for Gaara to realize that the moon is Vegeta's weakness.
Additionally, all your arguements either suggest that Gaara would somehow instantly think to attack the power ball before Vegeta could transform, or that Vegeta would just stand there and let Gaara attack the ball without intervening.