Travel dates between 1 September to 31 December. Tick the "lowest fare" option when searching and look for the dates with $0 fares, taxes will be added which is where the prices below come from. Luggage, meals, etc. cost extra on these sale fares.
To Ho Chi Minh City:
From Brisbane $437 return
From Sydney $431
From Melbourne $408
From Adelaide $435
From Perth $355
To Hanoi:
From Sydney $474
From Melbourne $451
There were still many dates available on all routes at time of posting.
Can you give some examples dates where cost is $200 return? I tried and its about $200 in taxes for each leg (~$400 return) even when selecting the $0 fares?