The <title> tag comes default in vbulletin templates. You don't need to add it to the headinclude template. For instance, this is in a default SHOWTHREAD template which makes it unique for every showthread page:
HTML Code:
<title>$thread[prefix_plain_html] $thread[title]<if condition="$pagenumber>1"> - <phrase 1="$pagenumber">$vbphrase[page_x]</phrase></if> - $vboptions[bbtitle]</title>
Why are you putting it into headinclude instead?