What little outside knowledge of these creatures that have been confirmed deal mostly with their nature and abilities. Vampires are creatures of the night. Their eyesight at night is keen, even surpasing the ability of drow night vision. As they are so tied to the night, so are they repulsed by the day. The very touch of sun upon their flesh burns them as surely as a flame would burn an ordinary man. Their eyes are as sensitive as a drow's to bright light.
Vampires are also creatures of remarkable stamina, possessing an extraordinary ability to heal. This stamina seems to be directly tied to the blood upon which they feed. A vampire derived of blood soon becomes so weak that he is unable to function. The very upkeep of their undead bodies costs them blood even if they do nothing else. Thus, vampires are in a constant search for more blood, only rarely becoming fully sated. Most alarmingly, they seem to favor humanoid victims.
Many of the ancient clerics of the world have long studied and a few have perfected means of turning a vampire back into a mortal. This method is by no means complete and there is always a vestige of vampiric abilities, most commonly the eyesight, left within the person. The vampire community has also made pacts with unholy gods to prevent the actions of these clerics. There are even stories of vampires going so far as to kill a cleric which has turned their "child" and turning the "child" back into a vampire.
Tread carefully in the world of the undead lest you become irrevocably changed.