Are you sure you added and not replaced the line?
There should be 2 classes loaded:
// load classes
require_once (JPATH_ADMINISTRATOR . DS . 'components' . DS . 'com_vminvoice' . DS . 'helpers' . DS . 'vminvoice.php');
require_once (JPATH_ADMINISTRATOR . DS . 'components' . DS . 'com_vminvoice' . DS . 'helpers' . DS . 'currencydisplay.php');
If you have both of them included, no error as you claim should appear.
For sure this should work in 1.2.1; please make sure that the files have been rewritten correctly. If unsure, check the permissions and apply "reinstall".
