Did you go through all the speech options with the vampire in Verdistis, and get him to admit he killed George and his reason for doing so?
Did you refuse to get George the Drudanae herb at the start of the game? That will break the murder quest, even if you later bring him his fix.
Tingalf's stash is a fair distance from the merchant's guild (check the sewer entrance in the middle of Ars Magicana). The group you encountered sounds like the one guarding the stash, though, in which case the tough one should have dropped a letter when defeated.
If you were chasing after Yarun and reached the Verdistis sewers, you went too far down.
Marcus-Tingalf-Trevor-Quest