The multi-vault feature seems very myopic and consumer focused and is definitely not very business or workgroup friendly.
In the 'real' world, especially the business world, users have just one single authorized account/credential.
For my eCommerce business, i have other people in the business who need to log into various supplier and wholesale sites using the company's assigned account.
Just like all our training materials, legal docs, OneNote notebooks, etc. All that collateral is in a shared folder structure on either OneDrive or OneDrive for Business.
All our people authenticate with their standard personal Microsoft account as is security best practices.
It should work the same was for the password Vault.
It should be able to be placed into the shared, but secured OneDrive location.
Then authorized people can log in with their personal credentials and access the Vault with the pre-shared key/master pwd.
Additionally, like @Toby Osborne stated....
As an consultant, I have credentials for many different clients. I agree that it would be fantastic to isolate them into dedicated Vaults for better client security.
But, I should be able to sync them all to ONE single cloud account for backup sharing across my devices. Making us have to have 30 separate cloud provider accounts does not seem to be a proper direction to take us. Having to have 30+ different accounts seems very counter to the whole modern SSO direction...