I have just left Vietnam and took 2 weeks going from north to south, even that i felt was a rush!
Although it can be done i would recommend consentrating on the north and do 2 nights in Ha Long Bay, as its amazing, Han Oi is a great city too and lots to do just outside.
Just my opinion
Tot of sharing my May '09 itinerary with you. Flights and hotel nights booked only. Still comparing the tours. The tours available are overwhelming and need sometime to choose but of course if you're not on any budget, the choices are easier made. Looking forward to this trip and I have to admit that before I did any read up, I was under the impression that its a 3 to 4 days place of visit but now I know it's not true! Two weeks is in fact insufficient if you consider the Central parts of Vietnam which I have totally left out.
Day1: Arrive Hanoi
Day3: Halong 2D/1N
Day6: Hanoi, overnight train to LaoCai
Day7: BacHa Market, 1N
Day8: Sapa, overnight train back to Hanoi
Day10: Saigon, domestic flight from Hanoi to Saigon
Day11: CuChi/CaoDai 1D tour
Day12: Mekong Delta 2D/1N
Day15: Fly home
Day 9: arrive in Hanoi (from Sapa) about 5:30am. you take the flight to Saigon in this morning to save time.