[According to this site,][1] if you transform into the Vampire Lord, all bounties you concer as a Vampire Lord will disappear as long as no one sees you exit Vampire Lord mode. This is probably because they just see the big, demonic, vampire, and not you, the scrawny red-eyed adventurerer

  [1]: http://elderscrolls.wikia.com/wiki/Thread:264026?useskin=oasis