[Modin]

you get nothing because when there's no result from your query, you should get nothing in your result, and therefore nothing for your rows.

Try using this code instead (also as Kirby said, don't use the 'WHERE 1' and have the 'or die' code)

$query = "SELECT * from vb3_user WHERE username like '$username'";
$result = mysql_query($query) or die('Query not successful: ' . mysql_error()); 
if ($result){
    echo('<p><strong><font color="#FF0000">! The username you selected is already being used. Please select another.</font></strong></p>');
}