I told you - in the VHost, put www.robin.blah.com down as a ServerAlias (alongside the current ServerName) of robin.blah.com.
Also - you dont need a document root or index file. In the VHost, you should be able to use a "Redirect Permanent" command. Saves a hit on the server and bandwidth to you. Check the Apache docs.
I'm gonna be honest - if you cant setup a simple vhost like that - are you capable of multi-site hosting like this?
How are you securing accounts? Are you chroot-ing users? Will apache be locked so PHP cant go up the folder heirarchy?
I'm also basing this on the fact that your site crashes horribly if you follow the link I gave in the first post. Either you were in one hell of a rush when writing that script or you're still learning how to make websites.
I dont mean to be rude of offensive - this is a very generous offer... But I get the impression you dont entirely know what you're doing.
EDIT:
Here is the redirect page you need...
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