Undead Vampires, ranging from Dracula, created by Bram Stoker, to the more urbane Lestat, created by Anne Rice, all share two basic characteristics. They are dead, and they need to ingest blood from mortals to continue to exist. They also share common traits with mortals. 
They stand, talk, move, run, and carry out all sorts of physical activities.
How do vampires carry out their active physical existences? Humans, mortals, have complex biological systems that provide energy to the muscles, tendons, bones, etc. that enable us to carry out physical activities. Vampires are dead: their hearts don't beat; their lungs don't breathe; and their former biological energy producing systems are no longer functioning. Where do their muscles get the energy? Dr. Katherine Ramsland wrote an entire book, The Science of Vampires, speculating about how vampires function and think. Dr. Ramsland could only speculate, because neither she nor anyone else has examined a "real undead vampire."
In the movies and in novels, vampires share blood with humans to turn them into vampires. How is this possible? Without hearts that beat, what keeps vampire blood flowing? Vampires are always depicted as being cold to the touch, quite logical since they are dead. Vampires are also depicted as being impervious to the cold. However, if they are cold-blooded creatures, and they are outside, or even inside, in freezing temperatures there blood would begin to thicken. That issue has never been addressed to the best of my knowledge. It's not a matter of unanswerable questions as much as it is a illogical possibilities.
We've also seen, or read about, vampires turning into some kind of dust or smoke. The transformation of matter from one form, a solid, into another form, gaseous, requires heat or some kind of chemical process. Modern science has no means by which it can almost instantaneously turn a solid object into a gaseous one with the exception of a nuclear explosion. There's a lot of bad science going on when it comes to the undead vampire. The stories of these creatures arise from folklore and are given life not by some magical process, but by the creative imagination of writers. The reader also has to participate, because more than a little suspension of disbelief is required to go along with the idea. However, judging by the current popularity of vampire books, movies, and now television shows, science and logic of the furthest things from the minds of readers and viewers.
