MIRKWOOD DISCUSSION

[Dark-Caranthir]

At last Ihave found something describing ME vampires

Whether it was bred from bird or beast that Melkor bred the evil bloodsucking Bat of Middle Earth, no tale tells. but in the First Age of the Sun, it is told how, in this winged form - made large and armed with talons of steel - Vampire spirits came into the service of Melkor.
In the quest of the Silmaril, Thuringwethil, the "woman of secret shadow", was a mighty Vampire and was the chief messenger to travel between Angband and Tol-in-Gauroth, where Sauron ruled the Werewolf legions. When Tol-in-Gauroth fell Sauron himself took on Vampire shape and fled. Once the sorcerous power of Sauron was broken, many evil enchantments were shattered. The shaping cloak that gave Thuringwethil the power to take Bat-shape fell from her, and the Vampire's fell spirit fled.

Guide to Tolkien

So maybe when Sauron rose again, the Vampires, who had been disconnected from their bodies and powers, regained them, and rallied around the banner of Sauron.