If you mean forum home, in the vB forumhome_loggedinuser template find:
Code:
<a href="member.php?$session[sessionurl]u=$loggedin[userid]" rel="nofollow">$loggedin[musername]</a>
And replace with the following:
Code:
<if condition="THIS_SCRIPT == 'index'">$loggedin[musername]<else /><a href="member.php?$session[sessionurl]u=$loggedin[userid]" rel="nofollow">$loggedin[musername]</a></if>
If you mean somewhere else that uses forumhome_loggedinuser, change the following to suit:
Code:
THIS_SCRIPT == 'index'