Assuming Kog's target has 30 MR (so, base magic resist) Kog deals the same %HP damage per hit as Vayne while his W is active. Obviously against higher MR values he's going to be dealing less.
No real conclusions drawn here, just an observation.
EDIT: To take the observation a step further: Kog and Vayne used to break even around 100 MR. Dropping the break even point down to base MR seems really odd to me.
I'd say the main separating point is that kog'maw's tank-melting potential is countered by his lack of escapes or survivability, neither of which is a problem Vayne players need to consider
????? You made no point or argument.
What even is the argument you're trying to make? You are just making a statement? That's like me coming here and saying Vel'Koz deals more true damage than Xerath.
If I can reach a Kog'maw though, I can kill him easily. If by some miracle I can actually reach a Vayne, she's just going to tumble away and be invisible half the time.
