We've found Expedia (USA) to be one of the best websites for searching for flights to and within Vietnam as they allow you to book flights with Vietnam Airlines online - no easy feat on VA's own website - as well as offering great cheap flights and connections to Hanoi and Ho Chi Minh City. For some reason Vietnam Airlines are only available on the US site though, so travellers from outside the USA will need to make their purchase in US Dollars.

More tips for finding cheap fares to Vietnam

• Choose unusual times to fly - Most people fly on weekends, making Fridays and Saturdays some of the most expensive flights available. By opting to depart mid-week you can make significant savings, sometimes hundreds of dollars. I find Tuesdays and Thursdays are often the best.
• Use flight search engines - Rather than just booking direct at an airline's website or travel agency, a flight search engine can often suggest some surprisingly cheap fares from airlines you may not have considered.
• Be flexible with destinations - Many people, myself included, like to enjoy the journey as much as the final destination. By electing to fly to a major transport hub such as Hong Kong (HKG), Singapore (SIN) or Bangkok (BKK) rather than direct to Vietnam you can make considerable savings, which will often pay for a few days hotel stay in that destination too. From these locations you can then use a budget airline to hop into Vietnam, or travel overland - the choice is yours!