Most, or at least some, of Kevin's resistences are High 1-C in potency, coming from the fact that he is able to resist all of the various effects of the Sea of Quanta. He also possess many powerful offensive smurf haxes, most notably Spatial Manipulation, Quantum Manipulation and probably many others that we still don't know for sure since he seems pretty casual most of the times he fight.
And I'm not really sure about this, but does characters that possess an high amount of Honkai Energy also emanates Honkai Radiation from their body? This would give him passive Corruption, but I'm not sure if Kevin specifically has it. The only characters that I know has this passive are the Herrschers like Sirin, not sure about the others.
Though, I'm not really sure if all of this is enough to put a fight against Vecna since I know very little about the character and he seems powerful.