I'm not sure if I understood you correctly... the place where the websites of the users are located is:
/srv/www/vhosts/domain.com/httpdocs
if you login with the user (domain.com) via ftp, normally you'll be right there... for SSH you need to chroot the ssh service to /srv/www/vhosts/domain.com so the one who logs in doesn't has access to higher directories, chrooting a service is not easy (you need at least a hardened kernel (grsecurity/SELinux.....) and take care what applications you put into the chroot (for example a /bin/sh in a chroot is definitely NO good idea)
kind regards
Rico Kerster