[Analogpoint]

One option would be on your homepage, pull the number straight from your database. If you don't want to run php on your homepage, do the iframe that calls a php file. The code could be:

mysql_connect('host', 'user', 'pass');
mysql_select_db('yourvbdatabase');

$r = mysql_query('select count(*) from vb_user'); // use the name of your user table
$r = mysql_fetch_array($r);
$registeredusercount = $r[0];

echo 'We have ' . $registeredusercount . ' users.';

This code doesn't take into account moderated/unverified users.