Sejs said:
Two points. The person doing the inviting has to have the authority to do so - a child arguably does not. I'd probably cut it at any adult to whom the building is home - mom, dad, grampa, uncle joe, etc. And I'm pretty sure the person doing the inviting has to do it of their own free will. If they're magically commanded it doesn't jive.
Not sure about the pocket rat thing. I'd be inclined to say that it wouldn't work. Riding isn't the same thing as invitation.
The rat idea was sort of an extreme situation, as I am uncertain as to how to cover that. Is the person who has been invited into the house, etc stopped as though they were the vampire? If they can still enter, then this is a work around for the vampire. If they cannot, then the vampire can cause chaos by simply riding in different people's pockets every now and then and discretely leaving in the chaos as those within think their loved one has become undead. Before long whole segments of the population may be under suspition of being vampire, even if they can later be shown to be able to enter the house, due to the fact that for a while they were as prohibitted as any vampire.
As for the invitation, I believe there are incidences in Dracula in which Lucy allows him into her room (in effect, inviting him) despite 1) not being the head of house (albeit still an adult, I admit), and 2) having been notably under Draculas control for several days. Also, there are old vampire stories in which the main horror of being dominated by the vampire is not being out of control, but that you who are dominated have just let into the house a creature that will likely slay you and all those you love. You did not invite the vampire willingly - only because you were under its control did you offer the invitation. So, IMCs, I would allow those dominated to be able to issue an invitation to the vampire dominating them - and thus allow the vampire into the dwelling.
Also of note, the Vampire, once invited, never again needs invitation into that building. This is common in most vampire stories, in fact, and is why the fear of accidentally (or unwillingly) inviting a vampire into the home is so tremendous. Once invited into the building, the vampire can return every night for the next few centuries if it wants to. Never again need it seek invitation. It could ignore the home, not entering it until the one that invited it is long dead and that man's great-grandchildren now live there. It will not need invitation, for it had such a century or more ago from one that had authority to give it.
Personally, I've always like focusing on the mythos and symbolism surronding the vampire. They were associated with fiends, granted, but their main association was with death and disease. In effect, by inviting a vampire into your building, you have invited Death into it. Once the building is tainted with its presense, only a cleansing might remove it - such as by having it re-consecrated.
Note that many buildings back then, upon completion, were blest or otherwise consecrated by the local priest. Inviting the vampire into the house could be considered similar to inviting sin into the soul. And while modern views allow more discretion into whether a sin is a sin if it was done unwillingly, medieval views did not. If someone sinned, it was a tarnish upon their soul. If they were blackmailed into doing so their soul is no less tarnished. I think the
might have been leniant if the sinner did not know that the act was a sin, but even then the soul was still tarnished - only not quite as much as might otherwise have been the case. They were far more strict back then than they are today, where relativism has weakened a lot of the concept of sin except in the most extreme cases.
So inviting a fiend / vampire into your home / soul - whether intentional (quite damning), under duress (mildly damning), or accidental (potentially damning - especially if efforts are not taken to redeem oneself after the brush with sin / death) - still 'tarnished' the dwelling, although from this perspective, I'd allow a re-consecration (via a Consecrate spell, perhaps?) to remove the 'taint' such that the vampire would again have to receive an invitation (akin to a 'reconciliation' that removes the taint of sin, in a way).