There is no vhost.conf by default, but you can create a vhost.conf file (vhost_ssl.conf for https) in a domain's conf dir and put any domain specific apache config in there. After creating the vhost.conf file you need to run /usr/local/psa/admin/websrvmng -a -v so vhost.conf will be included in the domain's httpd.include file.
If you want to set an option for all domains you could edit /etc/php.ini, but I like to create a separate .conf file in /etc/httpd/conf.d/ myself, which gets included in the main config automatically.