Considering that the yield beyond KUL and SIN was desastrous, I think it is much smarter to fly these birds to cities where the bird will be back in VIE the next day to operate another long range flight.
If you fly to Oz only 2-4 times a week you will lose the few C pax left who will migrate to airlines flying daily or even double daily. Although the TATL market is also not what it used to be yield wise, OS is experiencing healthy loads and unexpected traffic streams from Eastern Europe to the US, so it would not surprise me, if they add another US flight....ORD would be a logical fit.
OS would probably prefer India or China, but I do not know if the current bil. agreements allow them to increase their services.
One should also not forget that Malaysia pulled out of VIE, so KUL should be doable...and if I turn the wheel of speculation once more, TPE has to be mentioned as well...