Can't cure vampirism: Falion won't interact
Solution 1:
If you have not yet completed the Companions quests you can go and do the storyline. Around the middle you get lyncanthropy (turn into a werewolf) and vampirism will be replaced with a kick ass power to turn into a werewolf
Solution 2:
go with fast travel to any place , then come again with fast travel to Morthal. Then go to the down and you can talk to Falion.