I would say, no, because the same magic rule prompting the vampire to ask permission in the first place also requires the answer to be complete. Otherwise, why bother? They would dart inside even before you had a chance to say “you” with the excuse that since you were taking too long you probably were okay with it.
I agree with others in that it’s a course and part of a meal.
But when eaten alone, snacks are snacks or meals according to size. If it’s something small that won’t even fill a third of your appetite, in my books, that’s a snack. And in my books, precisely because of scarcity reasons you have less leveraging power to demand tax on a snack.