on a default vbulletin installation the logo is hot (link)
I'm assuming you have a custom style .. if that's the case it's a bit hard for us to tell you what to do; nevertheless, this is the code needed -add the 'header' template
HTML Code:
<a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a>