Due to the Potara effect, Vegito could outlast Gogeta in a fight. However, both of them will want to test the true limit of the other. Also, considering that they're both tricksters, the fight would last a very long time. With this being said, Gogeta's fusion timer most likely would've been up and both Goku and Vegeta would go back to normal. This leaves them in a more vulnerable state whilst Vegito has the power advantage. Although, if Goku and Vegeta the cooperate perfectly, they have a small chance to defeat Vegito. However, this is unlikely.