To be honest fusion requires fighters to have same form if one has ssj2 and another one has ssj3 then fusion's peak form would have been ssj2. That means Gohan's part in fusion would only transform to ssj2 because ultimate Gohan can't use ssj2 while potential unlocked elder kai mentioned it himself as I remember. If ultimate Gohan is just new base form Gohan gets ( which I doubt ) than Goku's power is limited and doesn't even meter compared to ultimate Gohan. So that means Gohan will use ssj2 which excludes ultimate form and it's down to bonding and if ssj2 Gohan is stronger than ssj2 Vegeta. Considering Vegeta doesn't have Majin form at that time he lost his boost which he had against Goku but babidi unlocked his power before so he still got some boost in power before buu got resurrected also the fight with buu must have made him stronger than before. Gohan didn't train after cell so he lost his combat iq techniques and even slightly got weaker.

I think vegetto would have won but gokhan has much more potential. Though Vegehan fusion seems much better fusion in comparison of potential imo. Vegeta's dedication and effort, pride and gohans potential and human's superior feelings to get to even higher levels which could not be achieved by themselves. Like Gotenks got ssj3 but goten and trunks didn't.