[kh99]

Interesting. Well, I think the forum being posted in is a hidden field in the newthread form, so there's no reason you couldn't have JS set that. It's hard to say what the best way to do it would be without spending time looking in to the details, but I'd say you could either use plugins using hooks in newthread.php to modify what it does, or else copy newthread.php to a new script and modify it. You say you don't want to duplicate newthread.php, so I suppose you could make a plugin that checks for a new value for $_REQUEST['do'] and then displays a new template that you create. Then you can use the existing code in newthread.php to process the submission.

As for the forum lookup, you could use a plugin on ajax_start, since that takes advantge of the existing ajax.php to process ajax requests (it's really just a script that's set up to load only the things that an ajax request would require). In any case you can look at that file to see how vbulletin responds to ajax calls (or maybe you know how you're going to do that already).