I had a problem with subdomain usage with ISPC3. I needed a subdomain with a separate document root but it kept showing the main domain.
I found out that this happens if you give a speciffic IP to the main domain but use the * for the subdomain(subdomain is a separate site here) while both are pointed to the same IP by DNS.
1) point the subdomain to a different IP (DNS A record)
2) set it explicitly to listen on the same IP in the "edit site" screen
3) unset the explicit IP for the main domain vhost in the "edit site" screen
Hope this will help someone. I lost half a day looking stupid at this problem
P.S. It is actually an apache quirk...but still may be usefull.