Ok, I finally found what was wrong.
On authmysqlrc, MYSQL_MAILDIR_FIELD must be the full path to the
maildir, not the relative path according to MYSQL_HOME_FIELD.
So, instead of
MYSQL_HOME_FIELD "/home/vmail"
MYSQL_MAILDIR_FIELD mail
It must be:
MYSQL_HOME_FIELD "/home/vmail"
MYSQL_MAILDIR_FIELD concat("/home/vmail/",mail)
Thank you for the guidance.