actually no, there is no difference whatsoever in the plugin to create for any version of vBulletin...
hook: showthread_getinfo
PHP Code:
if($threadid == 'XXX') $postorder = 'ASC'; # or 'DESC' depending on your needs
the logic behind the "no postorder" is that you usually start reading a thread when it started, and when you click the icon "go to last unread post", you are redirected where you were last time... if you force the order, the button will work, but it will require you to scroll back...