/home/vhosts/ or /var/www/
Solution 1:
According to Chapter 3 of the FHS (Filesystem Hierarchy Standard) 2.3, data for services provided by a server should go under /srv, but leaves the organization under it pretty much in charge of each specific system.
I would recommend /srv/www/<domain> or if the server is providing multiple services per domain something like /srv/<domain>/<service> (service being www, ftp, svn, etc).
Solution 2:
Red Hat and Debian use /var/www. If I was accessing an unfamiliar server, that is the first place I would look for a web server's document root.