I think a fundamental question has to be answered first, before a lot of these questions can be addressed:
Are we talking about vampires as an epidemiological or a supernatural phenomenon?
The powers and limits that make sense if we're talking about human beings who have a disease are different from the ones that make sense if we're talking about beings that fit into a certain religious/supernatural context.
Dracula could be harmed by holy water or a crucifix. Obviously that makes no sense if vampirism is just a disease.