Thank you!

For me it involves the same URL but it's great you put the IF ELSE in because that'll cover other people wanting that.

Now I need to be told where to PUT all that lovely code!

I guess a specific template for new posts but I'd be grateful to be told where to insert it.

Create a new plugin, select hook location "newthread_post_complete", make up a title (so you'll remember what it is later), and paste the above in the code box. Click on the "Yes" radio button at the bottom and save.

ETA: If you want to understand more about adding a plugin you can read about it here: http://www.vbulletin.com/docs/html/main/add_plugin