Sorry, I forgot to add that you'll also have to change the NameVirtualHosts line to:
NameVirtualHosts n.n.n.n
where n.n.n.n is your IP address. You may also need to map your virtual hostnames to the IP address of your host in /etc/hosts or elsewhere (depending on your name resolution setup):
/etc/hosts:
n.n.n.n myrealhostname.mydomain.com myrealhostname
www.domain1.com forum.domain1.com
Sorry for any confusion this caused.