I'm pretty sure that it will cost a lot more than $US 6k and that's not counting the cost of flights and accomodation in Australia (you will have to be here by 30 something weeks because airlines are funny about flying you about in the 3rd trimester).
If you're not comfortable with the standard of healthcare in Vietnam have you thought about Bangkok? They have very good hospitals, one of my friends had a stay/operation there for gynae problems and she was really satisfied with her level of care and the price (although she ended up having to haggle it lol). She had been using Australian Drs but she's more satisfied with the Thai system.