Nitrous Oxide is not banned nor illegal in Vietnam.
Balloons are available in 20% of bars in Da Nang and cost 100 k vnd (75 thb).
Highly addictive and the buzz appears to last just a few minutes.
I've watched people turn from normal into a mong like state hanging off their chairs, no conversation, falling asleep and to me it looks like their night is ruined....but they like it. So hey ho.