I don’t understand your goal. I see that the log message from imap-postlogin is misleading because it incorrectly includes the “Error” string, e.g.:
This happens because the imap-postlogin script writes that informational message to its stderr descriptor, which is then interpreted as a generic error.
Impersonation is required by Webtop and Imapsync to access a user’s mailbox. These applications need to know vmail’s password, which is obtained during installation through a privileged action.
If you change vmail’s password on the Mail side, you must reconfigure Webtop and Imapsync accordingly. I strongly recommend refraining from changing it.