using drush for upgrade drupal 6 to 7 - create alias problem
Although the Drupal root is a valid place to store alias files, Drush will not be able to find the alias records described there until the Drupal root as been specified.  Therefore, drush @sub status has a chicken-and-egg problem; you want to use @sub to specify the Drupal root, but Drush won't load the alias file that defines @sub until the Drupal site it is defined in is bootstrapped.

To resolve this problem, move your aliases file to $HOME/.drush/aliases.drushrc.php.  Once you do that, then your status command will work.

You may define your 'onward' alias either in $HOME/.drush/aliases.drushrc.php, or in /var/www/vhosts/domain.com/httpdocs/aliases.drushrc.php, per your preference.