4 days SGN
4 days DAD for Hue and Hoi An
3 days Hanoi
3 days Siem Reap

I just came home from 5 weeks in Vietnam (trip #8). As far as my preferences go, I would adjust that itinerary slightly since I'm not a big fan of HCMC or Hue, and I am a very big fan of Hanoi. You're missing some of the best of Vietnam in my opinion (Sapa, Dalat, Mai Chau and Quy Nhon). I was just in the small village of Bai Xep, a beach location just south of Quy Nhon - a total little paradise of a place because I had an ocean front room at Haven Vietnam Guesthouse.

I would actually skip HCMC entirely and spend those 3 days in Bai Xep (Quy Nhon airport), 4 days in Hoi An, 4 days in Hanoi and 3 days in Cambodia.