So I have a domain "testdomain.com".
Inside test domain.com's root is the following .htaccess:
Options +FollowSymLinks -Indexes
ErrorDocument 403 /launch/index.html
Deny From All
Allow From 192.168.1.2
When you visit anything other than root things work fine. Eg. if you visit http://testdomain.com/somedirfile.html
It will show the right error in /launch/........
There are a few ways of doing this and all basically involve using the reverse proxy or "ProxyPass" feature of Apache to accomplish it.
1.) Create a normal vhost and simply symlink the root directory of the site you want to mirror.
Eg. originalsite.com and newsite.com
You would symlink like this:
ln -s /vhosts/originalsite.com/httpdocs vhosts/originalsite.com/........