"Cult of the Vault" is a challenge from Tiny Tina's Assault on Dragon Keep DLC
The First symbol is found in the main area of Murderlin's Temple, just to the West of the Vending Machines. You will see a ladder there. Drop down to the bottom and head for the window overlooking the Forgotten Temple Arena. Look on the Eastern Wall to find the Vault Symbol.
The Second Symbol is inaccessible until you complete Rounds 1-5 of the Magic Circle of Slaughter. Once you can receive the side quest The Magic of Childhood and can access the The Nursery then you can find it. Head into the Nursery and turn to the right immediately. By the door you will find the second Vault Symbol.
Ranks and Rewards
Badass Ranks: 5