The porphyria explanation of vampires has always been pretty weak. It does not explain the main features of vampirism (i.e. drinks human blood) but only the peripheral ones (does not like sunlight). However, there is another non-supernatural explanation to explain many of the characteristics ascribed to vampires. Essentially, in communities that believed in some form of life-draining ghost, it was common to blame certain outbreaks of diseases like TB on them. One common procudre was to dig up and stake the dead person believed to be the cause of this misery, in the belief that it would lay their spirit.

Some of the physiological changes that can occur to a human body a couple of weeks after death include shrinkage of the gums (leading to the appearance of long teeth), ruddiness of the cheeks, bright redness of the lips, and re-liquification of the blood. Most people have never been in contact with a body this long after death, so they do not know this. In addition, gases often build up inside the chest and abdomen. Because of these changes, when a body is staked it can literally cause blood to spurt out, the mouth to open and a loud groan to escape. Thus, people who opened graves and staked bodies, who thought blood would be coagulated and groans would be impossible from a corpse, would believe that those bodies had indeed been vampires.

Various firsthand accounts suggest that this may be the source of some of the beliefs about vampires. Note that it cannot explain the initial belief in dead people who drain the life from the living. This is a very old ghost belief. But it may have given final form to many of the central ideas about vampirism: that life is drained by drinking blood (bright red lips and long teeth) and that the body itself is re-animated (groaning, bleeding). For more details, see Paul Barber's book, Vampires, Burial and Death.

A word on Reynardine. It was never, in its English form, a vampire story. Bert Lloyd tried to popularize a sort of werewolf interpretation, and most English revivalists bought that. It was Buffy Ste. Marie who called it a Vampire Story, and a few Brits (like Isla St. Clair) have now fallen for that.