There are 2 vampires in the game. The one responsible for George's murder is hiding in the wine merchant's cellar in Verdistis (the east house just north of the ducal inn). If you already killed him, then you should be able to report to Mardaneus if you went through the speech options first.
Verdistis screenshots and walkthrough
For the kidnapped boy in the dark cave, you need to sacrifice yourself to the vampire to save him. When the vampire bites you, you loose from 1 to 4 random stat points (usually 3), but there are more than enough stat or level bonuses in the game to compensate for that.