Completing the challenge for each area requires all of the hidden Vault symbols in that area to be interacted with, and these vary in number from a single Vault symbol in some areas to several in most. Once each of the symbols in a given area have been tagged in this manner, the Cult of the Vault challenge for that area is complete.
The symbols are usually well hidden, either concealed or in hard to reach places. They can be on any flat surface, be it wall, ceiling, or floor. The sizes of Vault symbols can vary also, with most being a large circle, while a few are extra large or quite small. They pulse in a silver sheen when tagged, to mark them as special symbols for the Cult of the Vault challenge.