Even though Goku has had a decent record in terms of befriending former Dragon Ball villains, the truth is, he isn’t actually the key to their redemption as there is another Z Fighter who has proven to be the common denominator in multiple villains becoming heroes.
The two greatest Dragon Ball heroes who began their careers as villains are unarguably Piccolo and Vegeta. Piccolo was introduced as the Demon Emperor, King Piccolo who was the embodiment of the evil that was expelled from Kami—a Namekian who became Earth’s god. When King Piccolo was killed by Goku, he produced an egg containing his own reincarnated body, which grew up to be the Piccolo fans know and love today. While Piccolo Jr., as he was initially known, continued in his ‘father's’ villainous footsteps at first, he eventually decided to carve his own path in life and not be bound to the limitations of what he was ‘supposed’ to be. Similarly, Vegeta made his debut upon his invasion of Earth, and like Piccolo, Vegeta meant to be the means of the planet’s destruction. However, after teaming up with the Z Fighters to fight Frieza, Vegeta decided to hang around the heroes as opposed to venturing out into the cosmos all alone—leading to Vegeta becoming a full-fledged hero. While the two most notable villains-to-heroes eventually got there on their own, there was one person who unarguably inspired their heroic journeys more than anyone—and that person is none other than Gohan.
In Dragon Ball chapter 274 by Akira Toriyama, Krillin and Gohan have teamed up with the villainous Vegeta after finding themselves on Planet Namek facing off against a team of the greatest warriors in the universe at that time: the Ginyu Force. During the fight, Krillin and Gohan decided to team up on one particular Ginyu Force member, a fighter named Guldo, while Vegeta tackled another one of the enemies. However, Gohan and Krillin ran into some issues as Guldo proved to be more powerful than even both of them combined could handle. Just as Guldo was about to launch a killing strike against Gohan, Vegeta abandons his fight and swoops in to save the half-Saiyan child. While Vegeta claims that he only saved Gohan because it was the smartest tactic, his display of ‘accidental’ heroism was actually the first time Vegeta ever made an effort to save the life of someone else on-panel–and that person just so happened to be Gohan.
A few Dragon Ball chapters earlier when Vegeta was just a nameless evil Saiyan in the minds of the Z Fighters who was on his way to Earth to cause havoc and chaos, Piccolo decided to take Gohan under his wing and train him in preparation for the Saiyans’ arrival. At that point in time, Piccolo was still committed to his villainous persona and claimed that he was only training Gohan to use him to kill the Saiyans. However, during their year-long training session, Piccolo started to feel something for Gohan, feelings which led Piccolo to give up his life of villainy forever and become a legitimate hero—a transition that reached its completion when Piccolo sacrificed his life to save Gohan’s.
While Gohan’s influence over Piccolo was much stronger and more evident, it can’t be denied that Gohan was instrumental in Vegeta’s slow metamorphosis into a hero from the despicable villain he was upon his debut—proving that Gohan, not Goku, seems to be the key to redeeming Dragon Ball villains.