Hmm,
I?ve VB 3.8.4 PL1 installed. To make work your Feature I changed your Code into
PHP Code:
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" /></a>
because I got
http://www.mysite.com/newthread.php?do=newthread&f=-1