Vampires are already dead and hence they cannot die. They can be destroyed, however. I tried to piece together the vampire physiology and have come up with this so far:
Vampires are undead creatures - sentient corpses. Magical evil keeps them animated. As undead creatures, they are immune to all things that relate to life: dying, breathing, bleeding, etc. They can be destroyed, however.
Vampires are traditionally killed by a stake through the heart. In other stories, other materials have worked as well. In
Ultraviolet, graphite bullets were effective and in the final episodes of
Buffy, a specialised steel weapon was effective. Graphite, like wood, is made of carbon. I'm guessing that the weapon in
Buffy was made from a steel high in carbon as well. So by bringing carbon to the heart of the vampire, it is destroyed, vapourised. Carbon is one of the elements of life. The heart is the muscle that pumps life through the body. Could this connection bring natural life back to the body, hence causing it to die as it should have (with a bit of acceleration in the decaying process)?
Sunlight and blacklights are common weapons against vampires as well. It seems that vampires react to ultraviolet energy. The sun is the largest source of this and consequently has the most devistating effect: spontaneous combustion. Could this be because a dead body does not produce any melonin, the natural agent that tints the skin and protects against sunburn? Almost all vampires are increadably pale.
Crosses can deter vampires. I'm a little dubious about this one. I don't believe a simple symbol could physically repel a vampire. A human could look at a "danger" sign and turn away, but only through sense, not some divine force. I believe that a cross would only turn a vampire if the cross had been properly blessed. Like with holy water. Holy water is just like water physically, except that it has been blessed. Throwing holy water on a vampire causes it to burn, yet normal water merely washes them. It is the divine nature of the blessing, carried in the symbol or water, that causes the damage.
Theser are just a few of my theories on vampires. Enjoy! <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
