Hi
I take it you live in Sydney, yes?
Your best option and cheapest option is Jetstar International. They will send you via Darwin but you might be able to pick up a great bargain on sale. You might be able to get it at around $500/600 return. Otherwise you can fly Vietnam airlines, but they are not cheap. It you are going to pay full price Jetstar International, then you might as well use Singapore/Malaysian/Vietnam airlines.
Do you want to go to Ho Chi Minh, Hanoi???
Mabye even Flying direct to Singapore/KL and then get a cheap AirAsia/Tiger flight for around $50 per way...