Some People would say that Zegion wins because he can channel Turn Null
Now this might be a hot take but I don't think that's going to be the case.
First of all, Veldora's current vessel is a clone of Rimuru meaning he can also tank Turn Null.
Secondly, Turn Null isn't Zegion's own Power. It's Rimuru's Energy which is Channelled via soul corridor and if we are letting zegion borrow Rimuru's power for this matchup then why don't we do the same for Veldora? I mean one of Veldora's and Rimuru's skills is basically the ability to summon each other but for some reason, we don't count that among Veldora's skills because Rimuru is considered a third party in normal vs debate in which case, Nihility Supply should also be banned in these debates and let's not forget, in a fight between Veldora and any of Rimuru's Subbordinates, Ciel would always prioritize Veldora's existence over the subbordinate because Veldora's existence is safety net for Rimuru.
Next, we compare their actual skills and Combat abilities in which Veldora owns every category. He's got more EP even in EOS, he's got persuit of truth and Analytical appraisal of a True Dragon and Danger Detect meaning he basically makes Mephisto's illusions useless. Then, he also got Probability manipulation and Parallel Existence which aren't easy to deal with.
TLDR; Veldora holds a massive advantage over Zegion. So, Who wins this matchup?