[Eosian]

Fine, add a plugin to hook showthread_query;

PHP Code:

$hook_query_joins .=  ' left join ' . TABLE_PREFIX  .'post parentpost on post.parentid = parentpost.postid ';
$hook_query_fields .= ', parentpost.username parentusername '; 


Now edit postbit_legacy or whatever you're using and add;

PHP Code:

// original line
                    {vb:raw post.title}
// add this after
                    <vb:if condition="$post['parentid'] AND $post['parentusername']">
                    (RE: {vb:raw post.parentusername})
                    </vb:if> 


Or a little snazzier;

PHP Code:

// original line
                    {vb:raw post.title}
// add this after
                    <vb:if condition="$post['parentid'] AND $post['parentusername']">
                    (RE: <a href="{vb:link thread, {vb:raw thread}, {vb:raw pageinfo_post}}#post{vb:raw post.parentid}">{vb:raw post.parentusername}</a>)
                    </vb:if>