to use a class in VM1.1 import the parser, import the class file and call the class
e.g.
// Load the virtuemart main parse code
if( file_exists(dirname(__FILE__).'/../../components/com_virtuemart/virtuemart_parser.php' )) {
require_once( dirname(__FILE__).'/../../components/com_virtuemart/virtuemart_parser.php' );
} else {
require_once( dirname(__FILE__).'/../components/com_virtuemart/virtuemart_parser.php' );
}
//then
require_once ( CLASSPATH. 'ps_user.php');
//then
$ps_user = new ps_user;
not tested but should work