With a little help from Brian over at VBadvanced.com I found that this could work by adding this line :
Code:
<input type="hidden" name="forumchoice[]" value="your_forum_id_here" />
so the final code I have put (to search by titles ONLY + search only a specific forum - with id=29 here -) is this :
Code:
<form action="forum/search.php?titleonly=1" method="post">
<input type="hidden" name="forumchoice[]" value="29" />
<input type="hidden" name="do" value="process" />
<input type="hidden" name="showposts" value="0" />
<input type="hidden" name="quicksearch" value="1" />
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="text" class="bginput" name="query" size="20"/> $gobutton<br />
</form>
thanx to MarcoH64 (vBulletin.org) and Brian (at VBadvanced.com)