Apache hosting two sites on the same domain

To be fairly honest with you, the best approch i can see for this would be if you setup your main domain normally like this:

<VirtualHost *:80>
    ServerName www.example.com
    DocumentRoot /www/domain 
</virtualhost>

Then you create a new virtualhost that will hold all domains you want to redirect like this:

<VirtualHost *:80>
    ServerName example.com
    ServerAlias foo.example.com bar.example.com others.example.com
    DocumentRoot /www/redirect_folder 
</virtualhost>

Inside that folder make a simple index.php page that summons the 301 so any domains hold in there will be redirect to your main domain with the 301 code.

<?
Header( "HTTP/1.1 301 Moved Permanently" );
Header( "Location: http://www.example.com" );
?> 

Why do you think it is better this way ?

This way you won't have to keep updating a bunch of places everytime you have a new domain to hold and redirct to your main domain and it won't be serving your users with the current name but will actually redirect them to your main domain in question.

If you are the server owner you can make it even better, you can put the 2nd virtualhost as the first virtualhost in your httpd.conf of vhost.conf file and whenever you hit the IP of your server it will lead you to the redirection page which will lead your users to the main domain in this case instead of having to set a bunch of ServerAlias you can just create the DNS A record for that given subdomain or domain leading to your IP and the server will take care of the rest.

In this last case all you would need for your virtual host would be:

<VirtualHost *:80>
    ServerName example.com
    DocumentRoot /www/redirect_folder 
</virtualhost>

as you dont need the ServerAlias since every and each request that hits your server IP will go to your first vhost.

In addition if you wanted to do this using .htaccess, it would be something like this i belive:

RewriteEngine on
rewritecond %{http_host} ^foo.example.com [nc]
rewriterule ^(.*)$ http://www.example.com/$1 [r=301,nc] 

You can do this with a RewriteRule preceded by a RewriteCond that checks the remote_addr (remember using % for the vars, not $:

<VirtualHost *:80>
  Servername somedomain.tld

  RewriteEngine On
  RewriteCond %{REMOTE_ADDR} 10.2.1.4
  RewriteRule ^(.*)$ /var/www/pages-1/$1

  RewriteCond %{REMOTE_ADDR} 10.3.0.
  RewriteRule ^(.*)$ /var/www/pages-2/$1

</VirtualHost>