The quick and dirty solution to this would be to just have a redirect page that bounces the user from
http://test.blah.com/ to
http://blah.com/test.html. You could do that in one of many ways - PHP, META Tag, Javascript, etc...
The elegant way and probably more preferred method would be to use mod_rewrite to transparently redirect the user. In your vhost block for that vhost you should have something that looks similar to this....
ServerName test.blah.com
RewriteEngine On
RewriteRule ^(.*)$ http://blah.com/test.html [R,L]
This will send any request for that VirtualHost to
http://blah.com/test.html