Funnily enough, assuming that Vampires can only enter a house if they are invited, being able and allowed to enter the house are, in this case, equivalent.
Would not asking if one can come in ask if it is possible or not, i.e. if the one is invited or not. So to ask "can I come in" is not to ask for permission, only what is possible and not, thus not the same?
Can I come in? => Have I been invited?
May I come in? => Can I have an invitation?