Actually even after healing yourself of your werewolf you can not become a vampire because you still retain your 100% resistance to disease.
That is untrue, on my current file I finished the Companions just so I could get the ancient nord and Dragur weapons and armor. After curing myself I caught the vampire desease and became a vampire. As far as I could tell I didn't have the 100% disease resistance after curing myself as a werewolf, so maybe that was a glitch? But you can definitely become a vampire after curing yourself as a werewolf.