How to redirect deleted forum pages to a forum on another domain with htaccess ?
 

We have split a database between an existing forum and a new forum on a new domain. Threads on the new forum were deleted on the old forum, but Google still list them on the old forum. We tried to redirect the pages using "301 redirect" in a .htaccess file. Although this works fine for html pages, it doesn't for PHP-built forum pages.

Here is how my htaccess file is written :

RewriteEngine On
Options +FollowSymLinks -Indexes
redirect 301 /forum/showthread.php?t=xxx http://www.newdomain.com/forum/showthread.php?t=xxx

Am I using the wrong redirection method ? Do I need to add something in the htaccess file ? Is there an option or a hack on vBulletin that allow to redirect some threads from a forum to another ?