Hmm in your chat file put something like this:
PHP Code:
if(!strpos($_SERVER['HTTP_REFFERRER'], 'forums.domain.com'))
{
$extra_js = '<meta http-equiv="refresh" content="3;URL=http://forums.domain.com/">';
}
Then in your chat template or whatever, put $extra_js in the <head></head> section of the template

If you don't run on templates, do a little digging within the files for the main chat page.
Also bare in mind referrers can be spoofed very easily, so this isn't really a reliable method. You might want to look at investing in a chat facility that does have actual vB integration.