I think the spell is based around trust, the vampire has to convince you to trust them enough (either through charm or trickery) to invite them into your private living space. I think an outhouse would work since there's a major expectation of privacy there.

I think a warrant would help the vampire convince people to trust them (a judge trusted them to execute their court order) but probably wouldn't be enough on it's own. Because it's not about convincing someone else to say it's ok to enter your home, they have to convince you to give them permission. But the vampire could say "I have been ordered by the court to search your house, may I come in?" They'd have to wait until you've read the warrant, maybe call your lawyer, and your lawyer would tell you that you have to let them in. Then you'd likely give permission to the vampire to enter your house because your lawyer told you to. Because lawyers are just another kind of blood sucker, aren't they?