Vaults are an idea brought to mind by CaptainSparklez, for reasons of too much gear or needing a safe spot when it comes time for thepurge.
The vaults are a storage room anywhere in the world. Every character may have one, and it's location is to be general knowledge. They're supposed to consist of a combination lock or code, leading into a sealed room containing everyone's most valuable items. Nobody except for the owner of the room is permitted to break or place blocks within the confines of the vault, and any indirect destruction of blocks (Crepper explosions, TNT, pistons, etc.) will end in trial.
It is forbidden to have a vault that is impossible to enter without breaking blocks, and if that rule is not followed, it is permitted to break blocks.
The Wizards are a known source of building structures and doing Redstone for large deposits of diamonds. In the early days of Mianite the wizards were hired to build vaults for the players known as SynHD and TheOpTicNadeShot. The wizards failed on their first attempt as both vaults could a easily accessed by random button mashing. The wizards were reluctant to fix this issue.
A vault is not required but is recomended. There is no rule about having more than one vault but it is required the location to your vault is known and accessable.