<DirectoryMatch /var/www/vhosts/(.*)/httpdocs/>
        php_admin_value open_basedir none
</DirectoryMatch>

If you want to remove all of the open_basedir restrictions for all sites in Plesk, simply create a file called /etc/httpd/conf.d/zzz_openbasedir_removal.conf and add this text within it (Be sure you trust your users.) :

Remove open_basedir restriction for all sites in Plesk

Apache