What are all of Wordpress' public/direct entry-points?
Here is a list of entry points based on require( 'wp-load.php' );
https://github.com/szepeviktor/WPHW/blob/master/wp-entry-points.md
And a Trac ticket about their deficiencieshttps://core.trac.wordpress.org/ticket/28364
So far, the following fixes my issue:
ln -s /var/www/vhosts/mainsite/httpdocs/index.php /var/www/vhosts/subsite/httpdocs/index.phpln -s /var/www/vhosts/mainsite/httpdocs/wp-login.php /var/www/vhosts/subsite/httpdocs/wp-login.phpln -s /var/www/vhosts/mainsite/httpdocs/wp-admin /var/www/vhosts/subsite/httpdocs/wp-adminln -s /var/www/vhosts/mainsite/httpdocs/wp-includes /var/www/vhosts/subsite/httpdocs/wp-includes