Apache: Forward Proxy Via Virtualhost

I've set up a forward proxy within a VirtualHost (see below) on Apache 2.2.11. I then browse using mydomain.com:80 as the proxy - I've also tried using the IP address of the VirtualHost xxx.xxx.xxx.xxx:80. It works fine, the only problem is that in both cases the server's main IP address is always displayed at the other end. Is there a way to make it so that the individual IP of the VirtualHost is shown instead?

I've tested the proxy via other VirtualHosts on the server and it doesn't work, so I know that it's being properly channelled through this one.

NameVirtualHost xxx.xxx.xxx.xxx:80
<VirtualHost xxx.xxx.xxx.xxx:80>
   ServerName mydomain.com
   DocumentRoot /xxx/xxx/public_html
   ServerAdmin xxx@xxx.com
   ProxyRequests On
   ProxyVia On
   <Proxy *>
      Order deny,allow
      Allow from xxx.xxx.xxx.xxx
   </Proxy>
</VirtualHost>

You should use the ProxyPass and ProxyPassReverse directives to get this working rather than the Proxy directive.

mod_proxy - Apache HTTP Server

Thanks for the reply, I tried your suggestion (see below). This produces a mirror of the external site (at http://mysite.com/mirror/\), but the main IP of the server (instead of the VirtualHost IP) is still showing at the other end - I'm talking about the REMOTE_ADDR environment variable. I want it to be the individual IP address of the VirtualHost instead of the server's main/global IP.

NameVirtualHost xxx.xxx.xxx.xxx:80
<VirtualHost xxx.xxx.xxx.xxx:80> # I want this IP address to show as the REMOTE_ADDR
   ServerName mydomain.com
   DocumentRoot /xxx/xxx/public_html
   ServerAdmin xxx@xxx.com
   ProxyPass /mirror/ http://externalsite.com/
   ProxyPassReverse /mirror/ http://externalsite.com/
   <Proxy *>
      Order deny,allow
      Allow from xxx.xxx.xxx.xxx
   </Proxy>
</VirtualHost>