Instead of using fan service moments, Dragon Ball Super is NEEDED to solve the eternal mystery of Dragon Ball Z.
When the franchise has been around for as long as Dragon Ball, there have been some mysteries that have bothered people. Although not very significant in the plot, sometimes the audience just wants answers. Interestingly, Dragon Ball Super finds itself in the unusual position of having to solve the remaining mysteries of Dragon Ball Z to advance its plot. In the battle against Zamasu, the question of how Vegito’s permanent Fusion was finally answered.
During the Buu Saga, Majin Buu managed to absorb Super Saiyan 3 Gotenks before turning Gohan into his toy. Things got so dire that Elder Kai gave his life force to Goku so the mighty Saiyan could join the fight. When Elder Kai quickly dismissed Goku’s idea of using the Metamoran Fusion Dance with Gohan, tactfully noting that they wouldn’t have time to study it, he suggested using Fusion via Potara Earrings instead.
Earrings worn by Supreme Kai were not just decorations — they were a more efficient means of Merging. When two creatures each place one earring in opposite ears, they immediately, even if they don’t want it, join together. Goku, who was only familiar with Metamoran Fusion, which lasted 30 minutes, asked Elder Kai how long Potara Fusion lasted. To his surprise, he was told that Fusion had no time limit, once joined together, the participants could never part.
After Goku returns to Earth, his detached parenting style comes back to bite him, as apparently, he and Gohan have never played catch before, like the earrings, he threw at his son sailed over poor Gohan’s head. Once he failed to fuse with Gohan, who was immediately absorbed by Buu, Goku almost fused with Hercule Satan until Vegeta appeared. Goku manages to convince Vegeta to fuse with him, easily leaving that it is permanent until it is too late to return. The newborn Vegito fights Buu before being intentionally absorbed so he can save everyone that Buu has consumed.
However, when Vegito releases his protective barrier inside Buu’s body, the Fusion ends as Goku and Vegeta are forcibly separated. Goku is confused while Vegeta believes that they are lucky. Although they briefly speculated that perhaps Fusion was not working within Buu’s body, the matter was never discussed again, not even by Kai. The idea that Fusion isn’t working inside Buu’s body isn’t traced, as Gotenks‘ Fusion lasted the full 30 minutes Buu spent using Gohan’s face as a resting place for his fists.
It wasn’t until the Zamasu arc in Dragon Ball Super that the issue of how Vegito was separated was addressed. After Goku Black and Zamasu use their own Potara to Fuse, Goku declares that he and Vegeta will follow suit. Interestingly, he doesn’t seem swayed by the prospect of it being permanent, just wondering why it’s not the first time. When asked, Gowasu, the 10th Universe Supreme Kai, explained that the Merger was only permanent if the Supreme Kai was involved, otherwise it would only last for an hour.
This retroactively explains why Vegito got separated inside Buu’s body — his time limit had run out. It’s clear that Elder Kai was only wrong when he told Goku it was permanent, but it’s also unclear why he didn’t know this. Despite being trapped inside Sword Z for thousands of years, flashbacks show that he knew about Potara’s powers beforehand when he tried to prevent an old mage from wearing one of his earrings. The mystery now was why only certain Supreme Kai seemed to know the full extent of the Potara Earring’s power. Unfortunately, the one-hour time limit ended up being a moot point when Vegito fought Zamasu and disappeared after a measly 10 minutes.