Despite the fact that Goku has a reputation for consistently being better than Vegeta, the Saiyan Prince has actually bested Kakarot a number of times–including one DBZ battle that was perfectly flipped by Gohan, who unwittingly avenged his father’s defeat in the most heartbreaking way.
Vegeta’s first win over Goku actually happened during their first-ever battle in Dragon Ball Z’s Saiyan Saga. Sure, Vegeta would lose to the Z-Fighters in the end, but he clearly beat Goku one-on-one. Another time Vegeta beat Goku was during their training-session battle on Lord Beerus’ world in the movie Dragon Ball Super: Super Hero. While these two examples certainly add to Vegeta’s proverbial score-card, there is one victory that stands out above all the others, even if the fight was won in the most dishonorable way possible: Goku vs Majin Vegeta. For this fight, Vegeta first received a massive power-up while also having all his inhibitions removed after he allowed himself to be corrupted by the villainous sorcerer, Babidi. The battle itself was epic and has since gone down in history as perhaps the best fight between Goku and Vegeta in history–but Vegeta only won after he tricked Goku into letting his guard down and then knocked him unconscious. So, technically Vegeta won, but it didn’t feel very victorious on his part. However, despite how it felt, Goku did lose to Vegeta during that fight–a loss that Gohan would avenge in Dragon Ball GT.
In Dragon Ball GT season 1, episode 27 (written by Daisuke Yajima, directed by Osamu Kasai, produced by Toei Animation), Goku, Trunks, and Pan have found the last Black Star Dragon Ball and are making their return trip back to Earth now that this intergalactic mission has been successfully completed. Meanwhile, back on Earth, Gohan and Vegeta are plagued with the presence of an unknown villain with immense power–one of GT's strongest Machine Mutants known as Baby. Baby has the power to possess people against their will and take over their minds, all while puppeteering their bodies and using their power as his own. In this episode, Baby has possessed Gohan and is using the half-Saiyan’s body to fight Vegeta–and in the end, Vegeta loses.
Gohan Beat Vegeta the Same Way Vegeta Beat Goku
The fight between Gohan and Vegeta in this GT episode plays almost exactly like the battle between Goku and Majin Vegeta in DBZ. A corrupted Z-Fighter challenges a heroic ally for the sole purpose of proving their superiority. For Baby, beating and subsequently possessing Vegeta was personal as Baby was created by an alien race that was decimated by the Saiyans, so Baby has an inherent hatred for them. Given that Vegeta is the Prince of Saiyans, he was the perfect target for Baby’s revenge. However, by using Gohan’s body, Baby unwittingly allowed Gohan to carry out his own vengeance in a way that perfectly mirrored the previous fight between Goku and Majin Vegeta in almost every conceivable way.
While Gohan held no ill-will towards Vegeta for his past conflicts with his father, Baby’s usage of his body still created a scenario in which Gohan was able to avenge Goku’s defeat in the most perfect way–though because Gohan had no control over himself and effectively beat-down a good friend and loyal ally, this adrenaline-pumping DBZ revenge-battle was much more heartbreaking than exciting.