Hard to give you an exact answer without knowing the VB table structure, but the query would be something like
$date = strtotime('10/01/2010');
/* this is the date you are basing our query on
we make a timestamp of it for comparison */
$sql = "SELECT * FROM `vb_users_table` WHERE `joindate` <= '$date'";
those "seconds" are timestamps btw (technically seconds, but the correct term is timestamp).
example:
<?php
$date = strtotime('10/01/2010'); //1st oct 2010
$old = strtotime('3/9/2009'); // 9th mar 2009
$new = strtotime('3/9/2011') // 9th mar 2011
/* substitute $old with $new to see the effect */
if($old<=$date) {
echo date('d M Y', $old) . ' is before ' . date('d M Y', $date);
} else {
echo date('d M Y', $old) . ' is not before ' . date('d M Y', $date);
}