Wendy,
Just for Reference to see timing, here is my itinerary in Vietnam
Aug 22, arrive 10 pm local time Saigon/HCM city
Aug 23, 24, 25: See HCM city with friends; trip to Cu Chi tunnels; museums, etc. (I'm staying with friends in HCM city)
Aug 26: 8:20 am flight from HCM to Siem Reap. Arrive about 10 am. Afternoon and evening tour Angkor Wat
Aug27: Tour Angkor Thom; other areas around Siem Reap, boat trip on Tonle Sap
Aug 28: Tour Siem Reap; Flight at 7 pm to Danang. Arrive Danang about 11 pm; meet with hired driver and go to Hoian. Check into hotel in Hoian.
Aug 29: Tour Hoian/ancient city/Hoian Museum
Aug 30: Meet with hired Driver early am to drive to Hue. Stops at Cham's museum, Thien Mu pagoda. Check into hotel in Hue. Wander around city.
Aug. 31: Imperial city. Royal tomb of emperor and tiger arena in Hue.
Sept. 1: 11 am flight from Danang to Hanoi. Arrive noon. Walk old quarter; possible museum stop; shop.
Sept. 2: Continue looking around Hanoi/museum/French architecture. Night Train to Lao Cai.
Sept. 3: Arrive early am in Lao Cai. Meet with guide and hike into Hmong villages in Sapa area.
Sept. 4: Continued hiking in Sapa.
Sept. 5: Hike back to Lao Cai and catch night train to Hanoi.
Sept. 6: Arrive early am in Hanoi. Shopping/looking around city. Midnight flight to Tokyo.
Sept 7, 8, 9: Tokyo then back to US on the 9th.
As you can see; it is pretty packed for two weeks. Yet I have down time built in too. And it is not going to cover everything I'd want to see in Vietnam (HaLong Bay, the floating markets, Nha Trang, etc.) You could pack a lot in 3 weeks, but you will need to look at travel times, etc.