Those are interesting ideas, N_B! I rather like the one of Vladek having Dewin uphold the enchantment. It could be blackmail. Or just a good ol' honest threat of death. Or he could promise protection from thedude, which would help explain how Dewin has been able to stay under the radar for however many years; probably a mix of both threat and promise. It'd add depth to Dewin's character if he was under Vladek's thumb and would have to screw up the courage to assist the KotOS despite that.
I'm not so sure about the idea of having Vladek doing the enchantment. I mean, if he could get some expendable guy to do it for him, all the better for him, right? Less of a chance for him to be accused of treachery while still controlling the spell, and if it's discovered there's not much lost.