How many people these days share their linux machine with anyone else? If you're the only user, first point doesn't matter. History file is a concern, but can be mitigated e.g. setting HISTIGNORE
Depends on the situation. A Linux machine running as a server can still be accessed by multiple people. Also, a "user" could be a compromised process that can execute commands. Using stdin or a properly created file means one less possible attack vector.
Doesn't storing the secret in a file first violate half the point of this service? Once it's written to disk in plaintext it's just as recoverable as your shell history.
if you the sort to worry about vault's command line being a security risk, you shouldn't be managing your secrets from your wordpress host in the first place.
