There are very few international flight to Danang, so it is often necessary to connect to a local flight from Hanoi or Saigon.
National Vietnam Airlines: The flagship airline of Vietnam, you can easily book flights to Danang and Hue airports. From HCMC to Danang, the flight time is 70 minutes. From Saigon to Hue, the flight time is 80 minutes. Depending when you book, a one-way ticket averages between 800,000 VND to 1.4 million VND. Make sure you keep your luggage tag, as it is checked by security! Bookings: www.vietnamairlines.com.
Jet Star: This budget airline has options to Danang and Hue. Delays are notorious though, so keep this in mind if you have an international flight connection. The average price to either Danang or Hue is between 450,000 VND to 1.2 million VND. Bookings: www.jetstar.com.
International Silk Air: This airline provides an international option to Danang from Singapore, however you will stopover in Siem Reap. The transit is short (around 40 minutes) and the total flight and transit time works out to be 3hr 45mins. But why not use the stopover to explore the ancient Khmer city of Angkor Wat? Return tickets average between USD300 to 500. Bookings: www.silkair.com