Trying to get vampirism cure, so I gave a filled black soul gem to the guy.
He tells me to meet him at the ceremonial circle at dawn.
Tried various methods, stay in inn, wait till 04-06 am, then go there, he's not there.
Tried standing there and waiting 1 hour, 24 times, but he never shows up..
Bug? Or am I just doing some faulty.

Must be a bug because you just have to be there at the appropriate time. Try loading up a previous save.

Read: http://forums.steampowered.com/forums/showpost.php?p=26350493&postcount=7

Still can't seem to do it right, when is dawn?