Airtrack360
All Star
That's because that's what it took for Goku to "break his shell". Goku and Vegeta have always achieved their power-ups by different means. Case in point, Goku reached SS1 after his body was broken several times and his best friend was killed. Vegeta, on the other hand, pulled it off purely out of his hatred of Goku.
This actually is really interesting.
Goku had to do something that he doesn't do often in order to gain UI, Lose or be defeated. Basically meaning giving his all and it still not being enough which is rare in Goku's case because he always seems to have a plan or way to overcome a situation.
What will be Vegeta's unique way of gaining UI?
Vegeta is no stranger to defeat so I don't think it will come from that.
My guess is self sacrifice is what triggers it. Dropping his Saiyan Pride, desire to suprass Goku, and title of Prince of all Saiyans. Thinking only of his friends and family and their future.
This is something that would break Vegeta out of his "shell". but this is only based off of the speculation that this could be how UI is unlocked in general.