While it remains unclear who is stronger between Goku and Vegeta in Dragon Ball canon, Vegeta certainly had the advantage in their first battle as he effectively defeated Goku one-on-one–a victory that stemmed from one psychological attack Vegeta didn’t even know he launched.
Vegeta made his Dragon Ball debut after he and Nappa invaded Earth during the Saiyan Saga. That storyline followed the villainous Saiyans as they attempted to steal Earth’s Dragon Balls in an effort to use their wish-granting magic to attain immortality and then kill everyone on the planet, so they could sell the vacant world to the highest bidder. Unfortunately for the villainous Saiyans, the Z Fighters are there to stop them which results in Nappa’s explosive demise. When Vegeta proves he’s more than enough to beat the Earth’s strongest fighters, Goku returns from the grave and gives the Saiyan Prince a run for his money–that is, until Vegeta completely throws him off with a single comment.
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In Dragon Ball chapter 233 by Akira Toriyama, Vegeta transforms into his Great Ape form to more easily crush Goku and win the battle. When Goku sees Vegeta as a Great Ape, he immediately thinks that he was the one who killed his grandfather when Goku was a child as his grandpa was crushed by a giant beast and Goku never figured out who was responsible. Vegeta then tells Goku that every Saiyan born with a tail has the ability to turn into a Great Ape, thereby connecting the dots that Goku himself was responsible for killing his own grandfather. When Goku learns this, he is sent reeling emotionally, and while he seems to recover quickly from this information, his actions tell a different story.
Mere moments after Vegeta reveals the truth of Goku’s first accidental murder, the evil Saiyan breaks practically every bone in Goku’s body and leaves him utterly defenseless. In fact, by all counts, Vegeta won his fight against Goku fair and square using his superior strength and powerful transformation and is only defeated after one of the other Z Fighters launches a surprise attack and cuts off Vegeta’s tail, thereby reverting him back to his weakened humanoid form.
While Vegeta ends up having to retreat after the remaining Z Fighters use their combined might to defeat him, he inadvertently revealed how to beat Dragon Ball’s greatest hero: he got in his head. Goku would later learn how to strengthen his mind as well as his body, so it is unlikely that this would work a second time especially because there aren’t really any more haunting secrets hidden in Goku’s past, but it did seemingly work this time around and proves how Vegeta bested Goku in their first Dragon Ball battle.
