What luxury? Both Goku and Vegeta went in there to surpass the SSJ. They both had one year. Goku had never seen the USSJ before.Saiyan-Professor wrote:
If you look at the situation Vegeta and Trunks left the Room immediately and went to go take care of Cell. Kakarrot had the luxury to work through in and outs of the form because if Vegeta had not given into his nature the problem would have been solved. Trunks said that Vegeta refused to go 3rd Grade because he knew of its weaknesses, thus he gave some thought to the form. The reminder is just bad writing on Toriyama’s part.
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There is no evidence that he mastered it first, Vegeta could have done so but he did not have the advantage of being dead and in a dimension that would allow him to attain Super Saiyan 3, which is essentially worthless. In addition, Kakarrot achieved the advantage in Ki because of his environment as well.
Vegeta comes out with USSJ, Goku with FPSSJ.
Goku gave more thought into it than Vegeta. Goku was always superior in transformations compared to Vegeta.
This leads us to your second point:
Gohan could maintain the SSJ for a short time when he transformed for the 1st time before he passed out.
Therefore, there isn't anything saying that you have to have extra ki or unlimited ki in your universe to be able to reach a transformation with a huge strain.
The SSJ3 could only last for some minutes in the normal world.
It would be very possible for Vegeta to reach that form at Earth, but he didn't.
Gotenks even reached that form without the unlimited energy and Goku's enviroment.
Plus, Goku was a step ahead of Vegeta in the Buu saga. As the SSJ2 needs some training and the right amount of power, I guess that Goku reached it first due to his higher power, which he already had from the start or that 7 year span. Plus, his training was more efficient in the Otherworld.





