My frivolous little post the other day on how vampires cover their expenses was but a prelude to this, what I think is the major implausibility about vampires.
Let us ignore the biology geek's objection about whether such a large and vigorous creature can sustain itself on blood alone (and, if it can, on what quantity of blood??). I am prepared to accept that the vampire's diet is just 'magic': it doesn't necessarily get nutrition from blood in the same way that mortal creatures do; this is just a part of the way it is, a compulsion that it has to follow.
But vampires do drink blood. From human mortals. To the point of killing them. Quite often.
How often, exactly? Well, in some tales, like Dracula, the vampire can spread out his feasting on a single victim over several days, or even weeks. In others, it is suggested that some vampires try to limit their murder rate by feeding on animals... or by stealing human blood from hospital blood banks. But, in general, it looks as though they're killing someone at least once a month, possibly once a week; in some cases, considerably more often than that.
Now, even if a vampire takes great care in concealing the bodies of his victims, and preys only upon the lonely and destitute, people on the margins of society who are less likely to be reported missing.... well, even so, that rate of killing is probably going to become noticeable before long. Even in quite a large city. In a smaller community, the impact is going to be immediate.
In recent Swedish vampire film Let The Right One In there's a (highly unusual) murder, another attempted murder, a disappearance, and a (highly unusual) assault, all within the space of a few weeks, or perhaps just days, in one small town - in fact, all within the environs of a single housing estate - and the local people pretty soon realise they've got a serial killer on their hands (the world of this film seems to be one in which people don't know about vampires).
It can't really be any other way. Vampires have a very distinctive and conspicuous way of killing people. They don't usually take much care about concealing their victims' bodies. And they surpass the headcount of any regular serial killer in quite a short space of time.
It's not just the attention this would attract that bothers me. It's the demographic impact: even one vampire could exterminate a small community over the course of a few years.
And usually we have multilple vampires. Indeed, we conventionally have a situation where the victims of a vampire become vampires in turn. Some writers have realised that if this were invariably to happen, there would be an exponential growth in the number of vampires - and vampires would eventually completely replace the mortal human population (what would they eat then?); so, they've tried to find ways of limiting the conversion of victims into new vampires - usually a suggestion that it is somehow the vampire's choice whether a victim becomes undead or simply dies. Even so, most vampire stories do seem to have quite a high rate of conversions - vampires want company, vampires want sex, vampires want an entourage, vampires like the feeling of power involved in creating new members of their kind.
So, you have more and more vampires feeding on humans. Where is it going to end?
Well, if any normal laws of nature were in play, I suggest it would have ended in the extinction of both species long ago. Sorry, horror fans - vampirism is a potent symbol, but it's just not remotely plausible as a basis for a story.