A vampire is a demon species which inhabits and animates a human corpse. To create another of its kind, a vampire drains a human of blood and, when the victim is near death, causes him or her to drink some of the vampire's blood. The new vampire "rises" after the human has lain dead for an interval which seems to be a day or so. Their most noted characteristic is that to maintain their higher brain functions, they feed on the blood of mammals, specially humans.
The process which a human body undergoes during that day or so that it takes to be converted into a vampire body is similar to the process of petrification that exists in nature, with certain plants. When a tree or plant becomes petrified, what happens is that over the course of years, the original biological elements of its cells are very gradually replaced by mineral elements... so gradually that the infusing minerals take on the same shape and appearance as the original biological cells. In effect, the original biological cells act as a matrix, or as a sort of "molding", for the incoming mineral deposits, and what you end up with is a gradual build up of minerals in the exact shape of a tree trunk, or a plant of some sort.
This same process, vastly speeded up, is what happens when a human body is transformed by the vampiric "curse", or infection, or mutation. Supernatural necromantic energy flows into it from an extradimensional source and replaces all the original human organic matter... but it does it gradually, and in such a way as to mimic the original cell structure. And the organic matter is not completely replaced, but is only 99.9999 percent replaced. A tiny amount of the original human cellular matter remains as a template, spread molecularly thin throughout the newly generated, unnatural vampire tissue which is made up of necromantic otherdimensional energy in more or less solid form.
This process is why vampires do not have souls. Since souls are intangible psychic organs they are not duplicated by this sort of necromantic petrification process, and as the original, natural body was transformed and replaced, the original soul attached to that body loses its adhesion and undergoes the metaphysical equivalent of evaporation into the ether.
This necromantic petrification process, explains the supernatural and superhuman powers of the vampire. A body composed of an unearthly energy we cannot rationally define within our physics can and will, by definition, defy the requirements of physics as we understand them.
This demonic, necromantic energy that makes up most of the vampire body is extremely responsive to the animating consciousness' psychic impulses.
However, relatively 'young' vampires still remember very strongly what it was like to be alive, and they still respond as if alive. For example, while he a vampire's consciously aware of the fact that he doesn't 'breathe', or at least, doesn't have to, if he also subconsciously wants to smoke a cigarette while he's smoking that cigarette, the necromantic energy in the shape of his lungs 'works'. When two vampires want to have sex, their subconscious minds activate the appropriate anatomical parts, and, similarly, when a male vampire wants to have sex with a human female, his subconscious reactivates his reproductive system.
Of course, a reproductive system composed of necromantic energy is only a simulation, and no actual pregnancy would be possible without severe mystical intervention... but to all physical appearances, the male vampire would be perfectly functional. To a human lover who expected such things, there is even a convincing sensory illusion of an ejaculation, although the physical existence of the ejaculate is primarily ectoplasmic, and will just fade away or evaporate fairly quickly after the sex act is over.
Since an unnatural, extradimensional, demonic energy is the basis for vampire bodies, patterned after the human bodies they are replacing, virtually everything is possible. An unnatural energy does not have to follow the laws of our universe (and could behave entirely differently in another universe).
Since a vampire's necromantic energy body is more amenable to his own psychic manipulation, most of a vampire's strength is telekinetic and of the vampire's will, expressed through manipulation of the energy form he or she inhabits. This is why more highly motivated and ferocious vampires often seem to be faster and stronger and fiercer combatants than the dumber or more apathetic ones, and especially, why vampires seem so clumsy and vulnerable when they first get up out of their graves... their remaining human consciousness, having just awakened in a strangely transformed body, may not be fully attuned to the necromantic energy they now inhabit. They may also not be fully awake. Either way, they can't manipulate their bodies with their full capacity.
The reason vampires burst into flames when sunlight hits them is that Earthly sunlight contains some wavelength of radiation that is missing from the sunlight of 'demon dimensions', and that acts on necromantically energized 'flesh' the same way intense heat would act on ours. In fact, vampire flesh bursting into flames in sunlight is the Undead equivalent of radiation poisoning, significantly accelerated and condensed in time.
Vampires turn to dust when they are destroyed because most of their 'body' is actually 'dark energy'. When something happens to dispel that energy... and there seem to be two central nexuses of this energy, one in the brain, and one in the center of the chest... it vanishes, leaving behind only the remnants of the original human body... the 'dust' that settles to the ground. The energy's mimicry of the human body is so complete as to make the recreated 'heart' and 'brain' necessary to the continued coherency of the vampire body. A stake to the 'brain' would also kill a vampire, but the skull is generally too hard to make this feasible, and the chest generally makes a much better target anyway.
Wood is a mystically or supernaturally imbued material, in a way that plastic and metal are not. This is because wood retains some essential element of natural Earthly life that is inimical to otherwordly energies; an elemental force that plastics or metals do not have. Wood also stores within its cells, through photosynthesis, the same element that sunlight possesses, which destroys vampire flesh. Thus, when thrust into a nexus of necromantic energy, as exist in the center of the chest where the major circulatory system of the human body was, it instantly disrupts those energies, causing the seemingly solid energy of the vampire body to instantaneously disperse, and the relatively small amount of organic remains to crumble to the ground as dust.
Given that the brain is the most sophisticated, complex, and vital organ in the human body, a necromantically duplicated brain is an essential organ in the vampire body. The various nodes that command the energies of the various different portions of the body still lead into the brain, and the necromantic brain duplicate is still constantly active holding the necromantic energy patterns of the vampire body into a coherent matrix. Segregate the brain from the rest of the necromantic body and the energy flow is interrupted and thus, the necromantic energy almost instantly disperses... and again, the organic remnants of the original human body sifts to the ground as dust.
Because of magic necromantically imbued substances cast no reflections. As a result vampires do not reflect in mirrors and not only does vampire flesh not cast a reflection, but even the clothes vampires wear don't reflect because their clothes quickly become suffused with necromantic energy when a vampire wears them, and thus, take on the supernatural aspects of that energy.
Vampires only cast shadows from artificial light because artificial light, like other technological artifacts such as videotape, interacts with necromantic energy in ways not taken into account by the legends and literature about vampires. Vampires don't cast shadows in sunlight or moonlight, don't reflect in 'natural' objects like shiny surfaces, water, and glass... but do block artificial light, and make an impression on videotape stock.
Vampires cannot enter a private, 'owned' dwelling space without specific invitation. The imposed limitation is, in fact, so specific and powerful that vampiric flesh can interact with the 'no invitation' barrier as if it were a physically solid partition.
This has to do with the sensitivity of necromantically imbued material to psychic energy. People tend to universally regard their private quarters as havens, sanctum sanctorums, their safe places, their private territory, their dens... places where they are safe and secure, where no one else is allowed to come without specific invitation. Normal human bodies, natural to this world and material plane, can move freely within it. Vampiric flesh is composed of supernatural, otherworldly, extra-planar energy. It can move freely in any place where psychic barriers have not been put up against it, but in specific, limited areas that are held as 'private', it needs permission to enter.
Crosses (and other holy symbols) seem to affect vampires exactly the way heated metal would affect humans (forcing them to back away in pain from close proximity, leaving burns and brands on their flesh if they come into contact with them), and holy water works on them like a powerful acid. Certain religious organizations have the ability to invoke through prayerful incantation certain aspects and supernatural powers that are inimical to the necromantic energy vampires are composed of. The Catholic Church for instance, through prayer, is able to 'bless' certain objects with the power to disrupt, if only in the specific area touched, necromantic energy.
Fire is also harmful and even potentially lethal to vampires because fire is an elemental force of this material realm, and possesses certain natural mystical powers of its own against unnatural, alien intrusions into this plane from outside. In addition, necromantic substances are simply very flammable.
Vampire senses aren't like human senses. However, when a vampire brain picks up various stimuli and other data with its enhanced psychic perceptions, it translates them into familiar human terms... like aromas. But in fact, what vampires have is a sort of direct psychic perception of their immediate surroundings that, their brain interprets into a 'sense of smell'.
A vampire's limited shapeshifting capacity, to switch from normal human appearance to Demonic Happy Face is a very limited and specific power, one that they have little or no conscious control over at first but that simply happens when they're enraged, or about to feed; however, older vampires learn to switch from one 'face' to the other at will.