The vault is indestructable but can be captured by enemy units as it is controlled by whoever has the most military units in the area, because of this it is better to place the vault in a well defended area in order to prevent an enemy to get the advantage of the vault. It is useful since it does the work of several villagers so frees up some population space. In addition resources are usually running low by the Mythic Age and the Plenty Vault generates an infinite amount of resources.