Vampires are a race of creatures that exist outside our ecosystem. While in myth and literature, these creatures have several weaknesses; In Boktai and Lunar Knights, they are weak to nothing except sunlight. The vampires in the game can also control armies of Undead, creatures of darkness, that are also only weak against sunlight. Vampires are separate and distinct from Immortals in that unlike an Immortal, a vampire can not return over and over again using the power of Dark Matter. However, being a vampire does not preclude one from also being an immortal, as The Count is both an Immortal and a vampire.
Non-Immortal vampires do not appear in the series until Lunar Knights, at which point they became the main bosses within the world of the game.