Trying to include Vietnam in round the world itinerary.

[kleenupnow]

Hi I am trying to include Vietnam in my round the world itinerary using Qantas Frequent Flyer Classic Flight Rewards. I wish to fly from Madrid to Vietnam then onto Melbourne. Regardless of which airport I select in Vietnam I get the response that it is not available on the Classic Flight Rewards. Does anyone know why this is the case or is there a work around? Thanks in advance