After George is murdered you have to talk to Mardaneus to get the quest to look for his killer.
When you find the vampire you have to talk to him to find out why he murdered George.
There are 2 vampires. The one that killed George is in Verdistis. The one that you get the quest to kill is in the Dark Cave. The quest appears after you save the boy, or not save him. Be sure you got the right one.
