Air Asia is a Malaysian discount airline that is pretty decent and headquartered in Malaysia. I know they fly from Malaysia to various cities in and around SEA. They have a Thailand branch as well and so it will be cheap for you to fly from KL to BKK if you find that there are some flights direct from BKK to Ho Chi Minh City. You can book on the web at Airasia.com
They were supposed to be adding flights from Bangkok to Vietnam last year (maybe just Hanoi, not sure), that might be an option.
You can also try this Asia airfare search site:Air Gorilla