There are two vampires in the game. For the one in the dark cave north of Ars Magicana, you need Lockpick for the door or Telekinesis for the lever (level 2 for each).
Did you talk to the head of the merchant guild again? You should be able to tell him you already had Tingalf arrested. This quest is relatively easy to partially break;
Tingalf/Marcus/wagon questYarun should be in the merchant's guild, in a room to the south west.