I have been reading the end times novels and in one great battle the Blood Knights turned upon the Undead (Nagash) and joined Chaos. A unit of blood knights had been sent to slow the Chaos horde and was stuck north of a magical wall created by Empire which blocked the Horde temporarily. the Blood Knights sent were always seeking a new challenge and were easily swayed to join Khorne when offered eternal battle. This is as far as I know the only instance of vampires joining chaos. As it is in the official end times novels it is loreful. The leader of the blood knights, Walach Harkong, struck the emperor down, who survived but was unconscious for the rest of the battle. Walach in turn attacked Vlad who was there to fight Chaos. Vlad was beaten in single combat by Walach but Vlad used magic to control Walach's dragon and turned it against him. The rest of the Blood Dragons surived but were not mentioned again.

I think vampire chaos knights would be cool as immortal vampire blood knights are already a frightneningly powerul unit. With the blessings of Khorne and chaos weapons/armor they would be the most powerful Knights in the Warhammer World. Their leader could be a hero, and is a known dragon rider and was the current leader of the Blood Dragon family of vampires. He bested Vlad and Emperor Franz in single combat. Its a shame we haven't seen either him or his unit in total war. And i reiterate- LOREFUL!!!!! From the end time novels on Nagash

I'll paste below the entry from the end times wiki
source http://warhammerfantasy.wikia.com/wiki/The_End_Times
From The Last Stand of Heffengen (Early 2525)
On the eastern flanks, Akkorak the Crow of the Kul tribe lead the Chaos Knights of Kruld's Rampagers and the Marauder Horsemen of the Wildkin of Hraldar against the Talabecland armies. The Talabeclanders braced themselves, and amongst their ranks Morrstan, last Wizard of the Athemyst Order hurled a blast of purple death magic upon the Chaos riders, breaking their charge. Back at the center, Karl Franz came upon the Great Unclean One and struck his sword upon its skull, banishing him from this realm. Yet as the battle seemed won for the Imperials, trumpets blared to the north as a host of Blood Knights came down south.[1t]

As they came closer, it became clear that the Blood Knights were touched by Chaos, and soon these Vampires showed their new allegiance to Khorne by falling upon the Imperials and Undead alike. In the skies overhead, Karl Franz was suddenly assaulted by the Blood Knight's Grandmaster, Walach Harkon. In but a few minutes, the Emperor, the greatest stateman and general the Empire has ever known, was seen falling from the sky, a gash of blood upon his chest. As the entire army saw him plummet, like a raging wildfire, the entire army despaired and ran for their lives. Only the two Undead armies continued their battle until Walach was finally slain by Vlad himself. His death mattered little, for the Emperor was dead, and the Empire stood ready to crumble.[1t]