I mean, mythologically speaking, vampires care about the religion of man because god actually exists and literally defends the faithful from their predation. Vampires also aren’t lawyers, barring an exceptional sci-fi series, so the rules they operate by are mystical and holistic. They can’t enter a home because it is a threshold defined and empowered by those living within, not by the laws that declare their ownership.

Squatters would be safe so long as they saw the place they lived as home. Generally someone who lives within the home and therefore contributes to the threshold is required to invite them in, but sometimes agency can be granted to others (i.e. a doorman or friend), but agency cannot be taken (i.e. the state). So the 4-year-old can invite them in, because it is their house until their parents kick them out. They also can’t cross running water because it purifies evil, same with why they’re burned away by the sun.

That said, it’s not like there’s a clearly defined foundation for their operation, and modern authors have modified how vampires work numerous times, so it really varies on a case by case basis.