From what I understand, a vault is a heavily fortified location (usually on a small island) where a (or several) character's hearth fires are located. I'm guessing this is associated with those that commit crimes often, as it keeps them safe from someone force-summoning them at their HF via scents.
There are some stipulations - namely, that there must be enough space on the island for a ram to be built. In other words, it has to be POSSIBLE to bash your way in.
This should also answer your other thread, about what the 'moose heads' are - they are village claims.
