If Wampa's ability happens in the Compare Results step, who cancels first? If the defender can cancel the crit, Wampa doesn't get anything. If you go on initiative and Wampa doesn't have it, his ability could be rendered completely useless.
It's for this reason I am more compelled to believe it should happen in the Modify steps.
I really don't see him as a token sink. No one is likely to use any tokens when facing him unless they can actually cancel the Wampa crit before Wampa.