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
/vhosts/originalsite.com/httpdocs
You would symlink like this:
ln -s /vhosts/originalsite.com/httpdocs vhosts/originalsite.com/........
understanding /etc/aliases
*remember to apply changes you need to run "newaliases" after editing /etc/aliases
one thing I don't get is that it doesn't allow you to specify the whole e-mail address on the left-hand side
eg:
yourfullemail@domain.com: someotheremail@domain.com
postalias: warning: /etc/aliases, line 109: name must be local (if you try the above)
It works more like this:
your........