One carrier needs to look at another port in Vietnam, mainly DaNang, for direct flights from Oz - would differentiate and attract a lot who have no choice but to transfer in SGN
I thought about that too. DAD is certainly the third largest port in Vietnam and is substantial in size and with has a strong domestic network. Surprisingly, Australia-Vietnam services are very focused on SGN, with only limited SYD/MEL-HAN operations. Obviously geography plays a role in that, but also HCM City is just that much larger than Hanoi, and Southern Vietnam also has stronger VFR connections to Australia. However, DAD would likely cannibalise a lot of HAN traffic so the incumbents (e.g. VN) certainly won't be interested. The question is whether there would be a first mover advantage for QH and VJ? Unfortunately, I can't imagine that this would be a high yield route, which undermines any gain from a first mover advantage.
However, the elephant in the room is the new airport in HCM City. SGN and the new airport will operate simultaneously (at least for the first 5+ years). There is talk that all international traffic will transfer to the new airport and domestic traffic will be split. One suggestion is that only VN will be allowed domestic traffic at the new airport for the first few years. If this were to happen, I'd expect QH and VJ to shift a fair amount of their international capacity elsewhere.