The others more or less covered what I was going to say. However, I will give a little more info. Vampires generally go through an awakening at or around puberty in which their vampirism makes itself known. Some can go through this awakening later in life as well.
Vampirism is caused by a damaged chakra, or energy center of the body. This causes the individual to produce less energy than he or she normally would; therefore, they need to obtain energy from others. Some do this by feeding from the energy of emotions or aura energy. Theses vampires are known as Energy, Psychic, and Psionic vampires. Some of these energy vampires also feed from the energy released during sexual intercourse. Others, Sangurian Vampires, feed from the energy in blood. Keep in mind this type only drinks a small amount of blood when needed. Vampires are human themselves and need actual food to survive just like everyone else.