Here's a possibility which kind of stems off of Skylark's concept.
Have one of the dead nobles appear to the PCs. A soft blue light wakens one of them and the figure makes no aggressive actions and in fact seems to be in great pain (obviously the vampiric baron may have conducted some manner of ritual or used a particular item meant to prevent their sould from returning but this particular one was stronger than most, yet his time is limited).
If the PCs daudle or take too long to gather you can have him flicker a bit and seem to weaken. The apparition won't know a great deal about the situation, but depending on how they were slain he may be able to identify the troll attacker by a vague description at least. You could set a limit on questions say, 5. Limit the apparitions knowledge of the situation to the things you want them to know and reduce the number of available questions depending on how long it takes the PCs to awaken and gather.
At the end have the apparition shriek in pain and appear to be torn apart (be it in the manner that he was killed or simply a violent dissapation of the form).
That would set a kind of creepy tone while giving them a little bit of a lead I would think.
