I use ProxyCommand in edge-case devices where key auth is not an option and the password is not controlled by me. ProxyCommand points to a script the retrieves the password from the vault, puts it on the clipboard for pasting, reminds me via stderr it's done so, and then proxies the connection.
Interesting. I might have such a use case. Do you have anything about best practices on how to automate grabbing passwords from vaults? Cuz it seems to me that the vault needs to be kept open or keep the vault password somewhere on disk.