vegeta never hired an assassin to take his life as part of training,It's Goku willingness to train and learn from others that makes him stronger. Throughout the Dragon Ball series Goku has had numerous teachers which include Son Gohan, Roshi, Karin, Popo, Kami, the Yardrats, Kaio, and Whis. Most of the techniques he uses today such as the Genki Dama, Kamehameha, sensing ki, Kaio-ken, and Instant Transmission are a result of his student-like approach to martial arts.
Even during the last tournament, while Vegeta was fighting Hit, Goku was asking around trying to understand Hit's ability (something Vegeta would never do). Because of his unwillingness to learn from others, even if Vegeta had went last, he would have still lost. I think this is because he believes he has nothing to learn from those he perceives as weaker than him. He also has this weird belief that he has to do everything himself. A belief that has rarely benefited him.
