We went to Vietnam in November 2001. It's not the best period : it's the end of the monsoon in the northern and southern areas but not in the middle area. We had a cloudy weather spotted by sun while the middle area was under a typhoon.

We stayed only 15 days. I think Vietnam needs one week more at least. This is our trip :
   Day  1 : arriving at Hanoi
   Day  2 : Hanoi - Cat Ba Island in the Along Bay (tourist bus)
   Day  3 : Cat Ba Island, trek in a forest
   Day  4 : Cat Ba Island - Hanoi (tourist bus)
   Day  5 : Hanoi - Mai Chau (tourist bus)
   Day  6 : Mai Chau - Hanoi (tourist bus)
   Day  7 : Hanoi - Chua Huong (perfume pagoda) - Hanoi (tourist bus)
   Day  8 : Hanoi - Ho Chi Minh (plane)
   Day  9 : Ho Chi Minh - île de Phu Cuoc (plane)
   Day 10 : Phu Cuoc Island
   Day 11 : Phu Cuoc Island- Rach Gia (boat to Ba Hon, then bus)
   Day 12 : Rach Gia - Cantho (bus)
   Day 13 : Cantho - Ho Chi Minh (bus)
   Day 14 : departure from Ho Chi Minh to Paris