Why not make it possible to have multiple vaults per account?
one account → many vaults
That way you have separation of data, some people asked for vault in a vault feature or a “fake” vault, this would also solve this.
e.g Nested vaults to implement multiple security levels
"Fake" Vault for Plausible Deniability
I dont know how backend works, but could you add a new optional parameter to a login e.g number
then the server would get your login request and this number, number would be match with a vault so server know which vault to send.
It would also be a security improvement as not only you would need to guess password/login, but also correct number (unless someone used default)