I'm having a problem too.
1) I saw the vampire leave George's house.
2) Later, i stumbled across George's murdered body.
3) I informed Mardenus. Otho didn't want to talk about it.
4) I stumbled across the vampire whist invading some guy's cellar.
5) I killed the vampire.
6) When I talk to Mardenus, no option to expose the vampire is offered.
The quest log is telling me I should talk to Mardenus, but I can't. I've tried sleeping as a quick fix - nothing. I'm not sure, but I may have bypassed a step between 3 and 4, something like speaking to the Verdisit (?) Watch.
Does anyony have any thoughts?