The Vault comes with Plus. It holds everything of value that isn't a bank: the house, the car, the coins, the papers that prove you own any of it.
Why it isn't just another account
Accounts is a ledger. It answers what you have to spend, it updates constantly, and on Buhata Money it fills itself in from the bank. The Vault answers a slower question, which is what you own, and the answer changes a few times a year at most.
Keeping them apart means your running balance never quietly includes the value of your car, which is the kind of number that makes a budget look fine right up until you need cash.
What goes in one
An item has a kind, a name and a value, and you can attach the paperwork that goes with it. The value is yours to set. Nothing tries to guess what your car is worth, because a guess printed as a figure gets believed.
It feeds Net Worth
If you hold Buhata Money, what the Vault carries is part of the Net Worth line rather than a separate total you have to add up yourself. Without the add-on the Vault still works and still totals; it just doesn't draw the line over time.
Who sees it
Nobody but you. The Vault is personal, the way money is personal in Buhata, and the server refuses to write a financial record into a shared Space at all.