I'm assuming you can't find the vampire in the fifth dungeon, not that you can't find the fifth dungeon. Therefore, in the third part of the fifth dungeon, there will be a branch of three different colors you can go off on. If you go on the green one, you'll enter an intro hallway. After that, in the main room of the green area, go up the stairs and through the hallway to the right. Once you turn up and keep walking her first event should trigger, and afterwards you can enter her room to find the second one.