In the era of Dragon Ball Super, one particular transformation called Super Saiyan God exists. Goku was the first to unlock this transformation through an ancient Saiyan ritual specifically designed for this purpose. Interestingly, Dragon Ball GT suggests that Vegeta could have achieved this power before Goku but missed his opportunity.
In an early episode of Dragon Ball Super, a new character named Lord Beerus, the God of Destruction, had a premonition of his demise at the hands of a Super Saiyan God. Wanting to face this powerful being, Beerus traveled to Earth, the only planet where Saiyans still resided. However, he soon realized that the Saiyans had only unlocked the Super Saiyan 3 transformation, which paled in comparison to Beerus' own strength. Disappointed, Beerus demanded to fight a Super Saiyan God. To uncover the secrets of this transformation, the group summoned Shenron, who revealed that it could be achieved through a ritual involving the combined power of five Saiyans: Vegeta, Gohan, Goten, Trunks, and even Pan, who contributed through her mother Videl's pregnancy. Once the ritual was complete, Goku became the first known Super Saiyan God in the world of Dragon Ball.
Vegeta was One Saiyan Away from Becoming the First Super Saiyan God
In Dragon Ball GT season 1, episode 29, Baby, a powerful Machine Mutant, successfully takes control of Earth by infecting the minds of everyone on the planet. Using Vegeta's physical form, now known as Baby-Vegeta, he engages in a fierce battle with Goku, Trunks, and Pan who had returned to Earth after their search for the Black Star Dragon Balls. Despite Goku's immense power, he is overwhelmed by the combined strength of Baby and Vegeta. Determined to become even stronger, Baby-Vegeta calls upon his Saiyan slaves, including Gohan, Goten, Trunks, and Bulla, to join forces and offer their power to him. Unbeknownst to Baby-Vegeta, this inadvertently mirrors the situation in Dragon Ball Super where the Saiyans misunderstood the requirements for the Super Saiyan God ritual. However, unlike in Dragon Ball GT, they receive clarification from Whis. This highlights the precision needed for the ritual to succeed.
Although Baby-Vegeta did receive a significant power boost from the energy given to him by these four Saiyans, he fell short of attaining the transformation into Super Saiyan God by just one Saiyan. It should be noted that Dragon Ball GT takes place after the events of Dragon Ball Super in the official Dragon Ball timeline, so even if the ritual was accidentally successful, Vegeta wouldn't have been the first Super Saiyan God. However, since the GT series was released before Dragon Ball Super, Vegeta would have been the first Super Saiyan God known to fans, provided he didn't miss his opportunity.