It's mediated through societal convention like how a man cave or the garage "belongs" to the husband even though ownership of those two room is actually determined by who owns the deed of the house in which the rooms are located in. A cool edge case would be if the vampire gets invitation from the husband but the husband and wife immediately have a big fight afterwards, perhaps over inviting a vampire over, and agreed to divorce with the husband and wife doing the thing long-term couples that break up do of partitioning their home and sleeping in their own partition (ie the man sleeping on the couch). Does the vampire still has have access to the whole house or are they now restricted to just the husband's partition of the house?