Some of the most fun to be had in a fantasy RPG session is when your players discover the cities can be just as dangerous as the dungeons.
If you've got a bard in the party, that player could easily take the role of Dandelion, or any tavern regular the group has interacted with in the past could start the quest instead.
Rather than focusing on a series of combat set pieces, this quest will have the group on the trail of a serial killer — but it isn't lacking in classic locations, like a sewer. The morgue and brothel scenes serve as great ways to get characters with lesser-used skills a chance to shine, and it would be simple to work in NPCs the party already cares about as possible victims to save.
For the quest's resolution you could keep the vampire connection, or change it to any bad guy lurking in the background who is killing for any number of reasons connected to your existing campaign storyline.