Keep in mind though that the Qantas codeshare from Sydney on Vietnam Airlines *is* allowed with the Global Explorer fare though, just not the oneworld explorer fare.

As for cheap options, flying Tiger Airways from Singapore is probably as cheap as you're going to find www.tigerairways.com . They've had fares recently for SG$9.98 each way plus taxes to Hanoi or Ho Chi Minh city, so that's not to be sneezed at.