I went this summer for a month, very beautiful country, we went the otherway Ho Chi Minh to Hanoi, loved HCMC maybe as it was a little western, Mui Ne is very nice if your into Kite Surfing, or Wind surfing otherwise its quite quiet, Nha Trang is a good night out, the beach isn't great but the nights make up for it. A must is Hoi An, by far the most original city, best architecture nice food and cheap suits, one of the best beaches in Nam as well., we got a taxi to Hue (the awesome mountain road on TG) we visited a natural park place with natural pools which was cool, of the tourist route a little. Found Hanoi pretty stressful, as you have to be careful of any local to do with tourism, so we escaped to Sapa, near china, which was amazing, climbed the highest mountain in Indochina, 3143 meters, a must see!
Hope that helps