mod_rewrite all subdomains to one directory.

I am a newbie on the whole mod_rewrite so any help would be greatly appreciated!

The functionality I need is:

www.mydomain.com >> needs to point to a top level dir ( mydir/www/docs/ )
--This is setup as virtual server in apache and is fine.

all_subdomains.mydomain.com >> need to all point to only one specific sub dir ( mydir/www/docs/all_subdomainsgohere )
-- I have wildcard subdomains set up in DNS and ServerAlias *.mydomain.com

I can get the rewrite workinging initially with an .htaccess in mydir/www/docs/:

RewriteCond %{HTTP_HOST} ^[^.]+\.mydomain\.com$ [NC]
RewriteRule ^$ /all_subdomainsgohere/ [L]


BUT when my page code starts trying to load in other directories from here: mydir/www/docs/all_subdomainsgohere
I get page not found errors.

So if my index page trys to redirect to all_subdomains.mydomain.com/login/index.html

This doesn't work, I assume it is looking in the top level directory but am unsure how to correct...

Can anyone can shed some light on this?

Thanks
Michael