Vampires have remarkable recuperative powers and cannot be harmed by conventional weapons. They are also immune to all forms of human diseases and illnesses.
Only when moving about during the day or when resting in his coffin, the vampire is somewhat vulnerable and subject to physical harm.
Bullets and ordinary knives provide only momentary distractions, as the vampire can heal from such injuries within seconds. Moreover, the vampire does not feel pain from such conventional attacks.
However, a vampire could not grow back a severed portion of his or her body. Apart from periodic inconveniences, injuries like these would not critically impair a vampire's "life" or effectiveness.
The only substances which could cause a vampire pain are silver and, to a lesser extent, unless it penetrate the vampire's heart, wood. The surest way to kill a vampire was to drive a wooden stake or a silver blade through his or her heart.
Vampires can only be harmed by weapons made of the purest silver and even then, their supernatural restorative abilities allow them to recover very quickly.
Under optimum conditions (i.e. - when they have been feeding regularly), vampires never physically get tired.