Within the era of Dragon Ball Super, there exists a transformation known as Super Saiyan God, one that Goku unlocked before anyone else after being the chosen subject of an ancient Saiyan ritual designed to unlock this power–and based on the parameters of this ritual, Dragon Ball GT reveals that Vegeta could have become Dragon Ball’s first Super Saiyan God before unwittingly missing his chance to do so.
In the early episode of Dragon Ball Super, the sequel series introduced a God of Destruction named Lord Beerus who had a premonition of his ultimate demise by the hand of something known as a Super Saiyan God. So, Beerus flew to the one planet where Saiyans still existed, Earth, in an effort to smoke out this Super Saiyan God and challenge them to combat. However, when Beerus discovers that the strongest transformation the Saiyans have unlocked is Super Saiyan 3 (which was a joke compared to Beerus’ power), he becomes disappointed and demands to fight a Super Saiyan God. After summoning Shenron who reveals the secrets to unlocking this transformation, five Saiyans–Vegeta, Gohan, Goten, Trunks, and Pan (through the pregnant Videl)–joined hands and offered their power to Goku. When the ritual was complete, Goku became Dragon Ball’s first known Super Saiyan God.
Vegeta was One Saiyan Away from Becoming the First Super Saiyan God
In Dragon Ball GT season 1, episode 29 (written by Junki Takegami, directed by Osamu Kasai, produced by Toei Animation), Baby, an extremely powerful Machine Mutant within the GT era, has effectively conquered the Earth after infecting the minds of everyone on the planet, while his physical form possessed Vegeta. Vegeta–who was now fully taken over by Baby and dubbed Baby-Vegeta–engaged in brutal combat with Goku this episode after Goku, Trunks, and Pan returned to Earth following their universe-trekking search for the Black Star Dragon Balls. While Goku is incredibly powerful (especially at this point in the series) he wasn’t a match for the combined might of Baby and Vegeta, but that didn’t stop Baby-Vegeta from wanting to become even stronger before finishing Goku off for good. So, Baby-Vegeta called upon his Saiyan slaves to stand with him, join hands, and offer him their power–and the Saiyans who participated were Gohan, Goten, Trunks, and Bulla.
Even though he didn’t know it, Baby-Vegeta made himself the subject of the Super Saiyan God ritual–he just didn’t have enough Saiyans to make it work. Funny enough, this is the same situation the Saiyans from Dragon Ball Super found themselves in when they didn’t hear Shenron correctly regarding the ritual’s parameters. Originally, only Vegeta, Trunks, Goten, and Gohan offered Goku their energy as they thought only five Saiyans were needed in total, not excluding the subject–it wasn’t until Whis explained it again that they got it right. This could have been an example of meta-commentary, calling out this scene of Dragon Ball GT, but it was more likely just a way to show how precise the ritual had to be in order to work.
While Baby-Vegeta did receive a massive power-up from these four Saiyans giving him their energy, he was just one Saiyan short of unlocking Super Saiyan God (accidentally or otherwise). Though it is true that Dragon Ball GT takes place after the events of Dragon Ball Super in the official Dragon Ball timeline (meaning that technically Vegeta still wouldn’t have been the first Super Saiyan God, even if the ritual was an accidental success), the GT series was released first, meaning Vegeta would have been the first Super Saiyan God exposed to fans–that is, if he didn’t miss his chance.