To be able to receive email for the new virtual user, you need to create their Maildir directory, set the correct permissions on it, and then create the actual virtual mailbox.
In the working example below, replace $USER with your username, $VUSER with virtual user name, $DOMAIN with your domain, and $MAILDIR with the directory where the virtual user's email is to be stored. Note that a relative pathname in $MAILDIR will be resolved by vmail relative your home directory.
maildirmake $MAILDIR fsr setacl $MAILDIR $USER.daemon write vmail $DOMAIN add $VUSER $MAILDIR # Enter a password as prompted. vmail $DOMAIN list # You will see that the new account shows up.