Try this: add new plugin showthread_query
PHP Code:
$hook_query_fields .= " ,subscribethread.*";
$hook_query_joins .= " LEFT JOIN " . TABLE_PREFIX . "subscribethread AS subscribethread ON (subscribethread.threadid=post.threadid AND subscribethread.userid=post.userid)";
Then in postbit (or postbit_legacy) add this to everywhere.
PHP Code:
<vb:if condition="$post['subscribethreadid']">Subscribed</vb:if>