I quite agree with Burmans, but yours is such a short trip - in my opinion too short to be moving around so much. It would help if you give dates. Personally, I like the North far more than the South, and Sapa is probably the most photogenic location in the entire country, save Halong Bay.
I too feel that Hue is only interesting to a point, and if you are not a history buff, I would skip it. I'm also not big on Saigon - Hanoi is the real jewel in my opinion (along with Sapa and Hoi An). Transport will slow you down considerably (road travel can be not only slow, but damn dangerous in VN). The fact is, you really need another week or 10 days to see this country properly.
I also agree with Burmans about Hanoikids (and other student groups) - the people in Vietnam are far far far more interesting than the junks of Halong Bay! Reserve with Hanoikids, or Danangkids or Saigonhotpot at least a month in advance (each has a website with booking instructions).
I think you will enjoy this trip far more if you research, book and travel completely on your own. Local day tours are fine, but you don't need an expensive guided tour to have a fantastic experience in Vietnam. (hint -
travelfish.org has great hints for all of SEA)