It’s quite straight-forward. Create the directory /var/www/vhosts/.skel/0/conf and then create a file called vhost.conf in it containing something like this:
<directory /var/www/vhosts/vhosts/@domain_name@/httpdocs> php_admin_value open_basedir "/var/www/vhosts/@domain_name@:/tmp" </directory>
The domain being provisioned here gets referenced as @domain_name@.