Starting with Borderlands 2, Gearbox introduced a challenge for most areas called Cult of the Vault, which required players to find hidden vault symbols in the game's areas. These vault symbols are usually hidden in areas that make the player explore sections of the map that they would otherwise not even think of exploring. That challenge returns in the Pre-Sequel, with each main area in the game having at least one vault symbol to find, and up to five.
Note: The vault symbols are totally optional and do not need to be gotten in the order listed. However, some symbols will require you to be on or have finished certain missions, which will be included in the descriptions.