The problem with any physical force that prevents the vampire from entering is this: How does it suddenly stop working when some person speaks a few words? If Divine Power is out, Psychology is your best bet.

@Cyrus I can make a flow chart for ya :-) Yes, limitation => can't enter, but limitation +-> words spoken/intention expressed => limitation lifted =>can enter. That's what I'm looking for. Maybe voice recognition/intention reader tech...

If it can be a technological solution that doesn't apply to houses not equipped with one, you have a lot more options indeed. "Sure you can come in, let me enter the code to disable the UV floodlights."

Do your vampires have arithmomania too?

Indoor light pollution: LCD and smart phones emit too much blue to UV lights that is harmful to vampire.

@Cyrus: On the other hand, why would you need to disable the anti-vampire measures if you didn't know they were a vampire? Generally, vampires need to impersonate a human to get their invitation.

Agree with Cyrus. Fear. Vampires get their strength because they can make people afraid. If someone is comfortable and secure in their situation - as people likely feel in their home - fear stops working on them. What if people were as comfortable and confident in their minds. Then they would have to ask for permission to charm them too. Lol.

I'm looking for a scientific explanation - perhaps you should rephrase as a science fiction explanation, since the iterative process of observation and analysis is not possible for fictional settings.

If it has word recognition software running, what if the potential blood donor only speaks something it can't understand like Basque? or is dumb? Then do gestures work?

The problem is that this isn't a scientific trait of vampires in the first place; it's a religious one, borrowed directly from the idea that a demon can't get in unless you let it in.

I always thought it was a CLASS thing - as if breaking and entering was beneath them. Vampires = classy, zombies = gauche.

This answer worldbuilding.stackexchange.com/a/7119/592 from one of the linked questions also covers this question.

But that would just be dreadfully impolite, dear chap! You can't just go into people's houses willy-nilly, there's etiquette to it. Do you think vampires are animals? On the contrary, my dear friend! Do not let yourself be persuaded by such comical flicks as Buffy the Vampire Slayer - a vampire who is not a gentlemen will surely perish very soon indeed. Noöne can live in isolation, and there is just way too few vampires around to form a working society without humans. 99% of a vampires life is still just blending in with the crowd and watching opera.

Do your Vampire society try to stay hidden from the humans? If Yes, just make it a crime in your vampire society because that would make the humans ask too many questions if vampires were allowed to simple go to any house and kill and disappear with the people while leaving everything intact.

@Luaan Vampires in BtVS are subject to this limitation, so it doesn't make a very good counterexample as you attempt to use it.

The problem with a physical barrier is, why in the world would someone remove a barrier which was precisely intended to stop vamps? Unless, it's an equivalent to a secure, locked door. I think @Liesmith's legal solution is the most workable. Perhaps the treaty is not with humans, but with some 3rd party having military parity with vamps; thus the need for a treaty.

@DanHenderson I think you missed my point :) I'm aware that BtVS vampires are limited in such a way - it's just a "comical portrayal", because they are really mostly mindless beasts. A real gentleman vampire doesn't need a physical rule to prevent him from entering a house uninvited - he simply wouldn't do that, that just isn't a nice thing to do. Would you enter a house uninvited?

@DanHenderson Exactly. It's an ugly anti-vampire propaganda :P Look at the vampires that are "sort of good", like Angel - that's a lot better picture of your typical vampire. Isn't it curious how much of a gap there is between your run-of-the-mill vampire and the "leaders"?