While Vegeta was introduced as one of the most bloodthirsty villains in Dragon Ball he actually helped an eviler villain find redemption—something Goku is usually given credit for.
Upon his introduction, Piccolo wasn’t the heroic Z Fighter fans know him as today, but was arguably the most evil villain in the Dragon Ball universe. When Piccolo, who was originally known as King Piccolo, was successful in taking over Planet Earth, he instituted a set of laws which enforced humans to commit unspeakable atrocities towards one another. While villains like Cell and Frieza are usually noted as being the worst of the worst, they only commit horrible acts themselves and don’t actively force a normal person to go kill their neighbor’s entire family under the threat of unspeakable punishment—meaning those other villains might have been a more dangerous threat, but King Piccolo was easily more evil. Thankfully, Goku killed King Piccolo, though the villain didn’t stay dead as he produced an egg containing his own reincarnated form–the Piccolo fans know and love today. Upon his birth, Piccolo picked up right where his ‘father’ left off, but after meeting Vegeta, Piccolo realized that the only reason he became a villain in the first place was due to a simple case of mistaken identity.
In Dragon Ball chapter 214 by Akira Toriyama, Vegeta and Nappa come to Earth to eradicate the planet of all Indigenous life and sell the vacant world to the highest bidder, but not before finding and wishing upon the Dragon Balls to make themselves unstoppable. However, the Z Fighters were aware of the Saiyans’ impending arrival and prepared by using every bit of time they had to train and get strong enough to fight them. The situation even called for Piccolo, who was still considered a dangerous villain at the time, to join forces with his enemies to defeat this greater threat. When Vegeta looks at his opponents, he identifies all of them as mere Earthlings who don’t stand a chance against him. However, when Vegeta sees Piccolo, he immediately comments on the fact that a Namekian was fighting alongside the humans—a comment that utterly shakes Piccolo’s entire preconceived notion of his origins.
Vegeta is the One Who Redeemed Piccolo, Not Goku
Namekians are an alien species from the Planet Namek who sometimes have access to magical properties, including the ability to create Dragon Balls, and are a peaceful race (for the most part). At this point in Dragon Ball, Piccolo had no idea he was a Namekian, or even an alien for that matter. In his previous life, King Piccolo thought that he was a literal demon who came up from the depths of hell for the sole purpose of spreading evil across the world. So, his reincarnated form continued on that mission since, again, Piccolo thought he was a demon and spreading evil was his purpose. However, when Vegeta identified Piccolo as a Namekian, Piccolo was no longer beholden to the notion that he was just an entity of pure evil who couldn’t change if he wanted to—allowing him to write his own destiny.
Vegeta opened Piccolo’s eyes to the truth of his heritage and Piccolo actually did become a legitimate hero in the Dragon Ball universe. While Piccolo was training Gohan in preparation for the Saiyans’ arrival, he was still holding onto his villainous side and even thought of later using Gohan to take over the world. However, after Vegeta told Piccolo that he wasn’t a demon but a Namekian who was free to make his own decisions in terms of the trajectory of his life, Piccolo actually sacrificed himself to save Gohan which was his first truly heroic act. So, even though he didn’t mean to and was merely just stating the obvious, Vegeta, not Goku, actually redeemed Dragon Ball’s most evil villain.