you can configure 192.168.1.10 as a proxy for 192.168.1.20 for
accessing portal.mydom.com
this looks simiar to this:
<VirtualHost 192.168.1.10:80>
ServerName portal.mydom.com:80
ProxyPass / http://192.168.1.20/
ProxyPassReverse / http://192.168.1.20/
</VirtualHost>
similar with 443 (https)