pixel_player33822906 found this on another thread:
The Vault of Guardians is the first in a string of five locations. You will be getting the masks from locations 2, 3, 4, and 5 and bringing them back to the Vault. In other words, clear out the Vault of Guardians as far as you can, and then move to the next location on the Side Quest map, the Hidden Pyramid. There you will find the green mask. Bring the green mask back to the Vault of Guardians, clear out as much as you can, and then move to the Desert Scar to get the second mask. Bring that back to the Vault, clear out as much as you can, and repeat the process with the Golden Temple and the Rivers of Babylon. After that, you will have all four masks, and you can finish the Vault of Guardians.
