Vegito, Kefla, Zamasu(fused), etc. the only fusions that have ever won was the ones in the movies, that’s because they’re supposed to win, cuz it’s a movie, but what about the show?
Mostly because the villan in the show are more powerful and if they use more power while in fusion like against Zamasu they ran out of time even with buu he was Vegito was stronger but Being stronger than someone and beating them are two entirely different things. If both Buu and Vegito were to fight, the battle would initially be lopsided in Vegito’s favour with Vegito easily overpowering Kid Buu. However, the problem arises when the battle is prolonged. The problem with Buu is that he is extremely resilient. You just can’t take him down easily and the biggest flaw of vegito is thag he likes to toy with he opponents