I'm not sure why everyone's arguing about lying when the vampire has no reason whatsoever to lie, or even subvert the promise. He explicitly says the baby is of no threat to him, he believes the world will end in a few hours so it won't matter anyway, and he was the one who suggested the promise in the first place. And if he does subvert it (which is highly unlikely due to the wording), Durkon can say he's broken the promise (as there's no neutral adjudicator) and continue to show memories that distract or mislead.

But then again, this is the forum, and I've seen someone alignment-lawyering with the Gestapo, so I'm not sure why the argument should have anything to do with the comic anymore.