HTML attributes should be enclosed in double quotes. You should add the vBulletin session hash to handle users without cookies enabled. Also, you should use HTML entities ("&", instead of "&").
HTML Code:
<a href="search.php?$session[sessionurl]do=finduser&u=$bbuserinfo[userid]&starteronly=1">My Threads</a>
<a href="search.php?$session[sessionurl]do=finduser&u=$bbuserinfo[userid]">My Posts</a>