The nature of and effects of vampirism ( in my opinion ) are somewhat misunderstood.
Although it's not necessary it is extremely helpful to have a background in ritual magick, or at least it's theory to truly understand the entities known as vampires. For there is a great deal of light that it can shed on the subject.
First of all I'll try to describe the various levels of reality as presented by high magick ( aka cabalistic magick, ceremonial magick, ritual magick, etc.)
The divine world: this is the world of archetypes, an archetype is the root idea behind a certain thing and many things can come from a single archetype, ie. the archetype for something that you rest upon could ultimately manifest in the physical world as a chair, a bed, a rock, etc. Archetypes cannot in any way be visualized or in truth be thought of and my explanation falls very short of reality.
The mental world: this is the world in which archetypes take on a definate idea. In the above example the differentiation between a chair and a bed. This is the level where you can get an idea of what " something to rest upon", is.
The astral world: is the world in which an idea can be visualized and is the blue print for the manifestation in to physical reality. It should be noted that the astral plane can be diveded into an " upper " astral and a " lower " astral. The lower astral is called the etheric plane and it is extremely close to physical reality. In fact beings that exist at the etheric plane can somewhat interact with physical matter. This is also the level that poltergeist activity takes place or any occult force has a direct influence on matter.
The physical plane: this needs no explanation as it is the world we interact with every day that everyone is aware of. Now these worlds can be looked at from the position of a human creating something or " God ". The process is the same.
Now on to some vampire folklore. Traditionally vampires have some very ghost like qualities. Hollywood vampires are a different matter altogether and don't have a lot in common with folklore although there are some commonalities.
Let's start off with the powers of vampires. The ability to change into a " mist ". Etheric energy that is dense enough to be seen many times takes on the appearance of a vapor. It is impossible for a corpse to turn into a mist and back into a corpse either from the stand point of physics or current magickal theory.
Vampires can change shape into various animals. Once again this is an impossibile feat for a corpse to do from either standpoint. But it is possible for etheric energy to do, and is frequently done by magicians and shamans alike when they wish to assume another form.
Vampires cannot cross a threshold without being invited: From the standpoint of a physical corpse, it can't move at any rate. However from the standpoint of magick and in the case of an etheric revenant, many times a person has to "invite" something in for it to affect them. A manipulative magician has a difficult time using magick on someone who is totally uncooperative, ie. won't see them, won't let them into their home, has no dealings with them etc.. The same is true for a "spirit" of some sort, ( for the most part ).
A vampire drinks the blood of the living: In most cases of folklore there are no bites found on a victim although this is not the case 100% of the time. They seem to be the exception rather than the rule though. In most cases the victim wastes away and dies. Once again this fits in very well with an etheric revenant as they would drain the lifeforce of the victim and not blood.
Vampires can't cross running water: This is true for the older versions of hollywood vampires as well as etheric creatures and once again we find the answer in magick. Water dissipates etheric energy. This has been known in magick for countless years. I could go on for quite some time but I hope this shows that vampires are etheric creatures and not Physical ones.
Peace to all