The soul vault is a place to store limited equipment. Every item in the game is assigned a limit (from 2 to infinity) and when a character logs in, their equipment starts to count against our limits. If an item reaches the logged-in limit, you will be asked to wear a different item until one of these conditions is met: A player already using the item removes it. A player already using it dies. Or a player using it logs out.
A quick example: Every player in the game has the Strife Spear in their soul vault. Sheia logs in and wields one. Cato logs in and wields one. At this point, any player logging into the game wearing a Strife Spear will be placed in a safe vault room to adjust their equipment and wield a different weapon. Sheia logs out and now any player may wield a Strife Spear until there are 2 spears wielded in game.
Every MOB will now load all of their equipment all the time. This means you can go obtain the Strife Spear (if you know where to look) and keep it in your vault.