Can I say an unbiased answer without anyone jumping into my ass?
It is true that Vegito has few feats, I mean his appearance was very short, but what we saw was still very impressive. He was able to outclass Super Buuhan indicating that he is stronger than Ultimate Gohan.
IMO, while Ultimate Gohan lacks the speed, strength, and durability of Superman, the difference between them isn't that big, and Gohans power is considered limitless due to being a mystic.
If we said Ultimate Gohan vs PC Superman, my answer would be Gohan 5.2/10 at the very least. Gohan is the only DBZ unfused character who actually stands a chance against him.
The fused character from DBZ who stands a chance against PC Superman is absolutely Vegito, but this is based on powerscaling. How much does he win by? Again, his lack of feats don't give a good enough scale, but I can see the fight won by 5.5/10 at the least, because he has shown the potential to be as fast as Supermans combat speed, possibly even faster, and his durability should be enough to handle Supermans attacks.
The only reason why I would say Superman wins by 4.5/10 at the least, is because of three major factors; Energy, heat, space.
Supermans Energy output is practically unlimited. Vegito who still uses Ki is limited. If these two duked it out for a few hours, Vegito would be sweating and puffing long before Superman.
Heat. Superman has shown to fly through stars and through planets disregarding the plasma and magma. His heat visions intensity is also as hot and hotter than the core of the sun. If you're asking how that works, it's the same way a Power Station works. Superman doesn't just absorb solar energy and then use it, but his body makes the energy usable within his body, making the power output far higher.
With his heat vision, it is a laser, or photons if you will, that can't exactly be stopped unless the mass it's hitting is very solid. So unless Vegito has a super tough skull, he will be getting lobotomized.
and then there is the major factor of Space. Superman can not only fly through space with ease, but survive the vacuum, the radiation, and the deepening cold. It's been well established that Goku can't, and I'm still not sure about Vegeta, though space seems to work differently in DBZ than it does in DC.
How these three can affect the battle will determine whether Superman or Goku will win the battle.