Sorry to hear your experience,
I've recently used it in MEL a few weeks ago, which may have much more friendlier staff than their SYD counterparts - however I must agree that the VN representatives are very rude, even when speaking to Vietnamese people.
I received the confirmation email from Optiontown and immediately checked my booking through Sabre's 'Virtually There' website, where my itinerary was altered with J showing on the ticket - printed it out, and also the confirmation too and everything was fine at Check In.