Vampires consume blood, but don't require it to flow. Most vampires are biologically dead. Those of Draculian and Gabriallan descent are, although it can't be guaranteed for other types of vampire. As such, they look much like corpses would. As a corpse will acclimate itself to the temperature of it's environment - much like any other object - it would become incredibly hard to see through infrared. This makes infrared practically useless against vampires.