Vigilance is a one-handed Valyrian Steel sword that can be found in Oldtown. ACOK 5.0
This weapon is owned by House Hightower, and can be acquired by taking the town of Oldtown itself. Once the town is taken, and is ruled by the player, a scene will pop up where the game will ask if you want to take the sword, or leave it.
If the sword is taken, you will lose honour and Ser Baelor Hightower will have -100 relations towards you.
Vassal or king?
Note that you need to be an independent king to get the sword. One does not simply capture Oldtown to get the sword.