What appears to be a four-by-two rectangle on your mini-map is actually a three-dimensional labyrinth with seven levels. You can follow Overdune's instructions and use the right-hand rule. Just imagine keeping your right hand on the right wall. Keep moving from room to room, turning as necessary to keep your right hand on the wall until you come to the unique room. As you leave, use the left-hand rule by keeping your left-hand on the left wall at all times.
Actually, traveling to the family vault using the left-hand rule works as well. It's less confusing and a bit shorter if you start using it from the Dungeon entrance -- a bit longer if you just count from the top of the labyrinth. It does take you through more unique rooms, however, so you may have a bit more fighting as well as Experience Points and treasure.
Still, neither of these techniques will either get you to your goal the fastest or pass through all the rooms....