1. There are no real vampires. Cut and paste time, as it is too much work to type this out over and over so here goes. A brief discussion of the human digestive system and then the probable vampire population given an exponential growth rate should explain why vampires are not possible.
2. Vampires do not exist. Period. The human body is not designed to process large amounts of blood for nutrition. There is not enough protein, carbohydrates, and fats present in blood to maintain a complex creature such as Homo Sapiens or any theorized offshoot mutations. When a human ingests food it is first broken up by chewing, then churned up in the stomach with digestive juices to form a bolus called chyme. It then passes into the small intestine where it mixes with bile salts which continue breaking it down on a molecular basis, mostly affecting fats at this point. The broken down nutrients pass through the wall of the intestines and into the bloodstream where they are carried to each cell or stored for later use. Indigestible bulk continues through the intestines, turning a dark brown from the bile. Water is absorbed from this mass in the large intestine depending on the needs of the body - a well-hydrated person will usually have a softer stool than a dehydrated person will. Water also enters the bloodstream, and this is what helps to maintain blood pressure. The pressure tends to balance itself in a healthy person because the bloodstream goes through a formation in the kidney called the Loop of Henle, where the narrowing blood vessel forces excess water and cellular waste such as urea out through the cellular wall into the kidneys, where it is excreted through the ureters into the bladder, and then out of the body via the urethral passageway.
3. IMPORTANT - A person physically unable to process his own food for nutrition therefore also could not process blood - it's the same process. Ingested blood does not transmit directly to the veins anyway - it would be chemically broken down by the digestive system.
4. Theoretical ingestion of blood to supply these nutrients would therefore have to occur at least once a day, and would require the ingestion of the entire blood supply which could not happen as the stomach is far too small to hold that much liquid volume. Furthermore, such a mass would be difficult to pass thru the intestines as it has no fibrous bulk, would create an intestinal impaction, causing massive vomiting from the large concentration of iron present, and any "real" vampire would have to eventually expel the waste, which would come out as a black, tarry, smelly goo.
5. These humans that affect the whole "vampiric lifestyle" are NOT vampires. They are simply humans playing their own little game, in their own little fantasy world. I too, play my own little game, in the SCA, but mine is a game where the deeds that I do are determined by the strength of my arm and sword - I am a warrior, with just as much skill and ability as any warrior of ancient times. The difference is that I am claiming to be something physically possible: a warrior, and I prove it everytime I strap on my armor and walk onto a SCA battlefield. The so-called "vampires" are claiming to be something physically impossible: a walking corpse, and all they prove is that black Victorian clothing, a pair of false fangs, and a little makeup make for a good Halloween costume - it does not make you a vampire.
6. Also, if a vampire has to (per my earlier example) feed at least once a day, and if this results in a new vampire every time (per the original legends) then the original vampire creates 365 offspring in just 1 year. The first one he creates then sires 364 more that year, and the second sires 363, and so on and so forth, do the math - almost a million vampires in the first year alone! Reproducing at an exponential rate, even with today's global population, they would totally convert every human on the planet in less than five year's time! (In 1 week's time alone that would be 35 vampires, and by the end of the 2nd week there would be 7 more for each of the 35, for a total of 245 plus another 34 for the new ones, or a grand total at the end of 14 days of 279 vampires!) This falls therefore, under the logic of Occam's Razor - which states that when you have removed every impossible answer, whatever remains, no matter how improbable, must be the truth. Since there is no "vampiric plague" swarming the earth, the logical deduction is that they don't exist.
7. Point of clarification about "vampire" bats: vampire is simply the name we have given them because they do drink blood, same as a flea, mosquito, or spider. Are these creatures vampires? No. They are living creatures, not legendary monsters. Drinking blood does not make you a vampire anymore than eating raw meat makes you a werewolf, although it might make you a mosquito.
8. There is no "virus" that can rewrite your DNA that radically - you would simply die from massive systemic shock as your body failed on a molecular level. Vampires do NOT exist, and those that claim they do are in need of serious psychological counseling.
9. Tell you what - I will be camping outside at Cedar Lakes in Ripley, West Virginia during April, at a little SCA event called Blackstone Raids. My medieval pavilion will be clearly marked with a heraldic banner the same as my shield in the avatar - an emerald gryphon on a silver and sable gyronny. (A better pic of it is on my 360 page.) Any "real" vampire out there should therefore make a nice little snack out of me if they really exist.
I'm calling their bluff.
I guarantee you I will still be posting here this time next year.
Edit: My karma would be more seriously harmed if I didn't try to educate others - I'm a Nurse and teaching is one of our callings.