Spent this Decemeber/January there. Awesome. Travelled North to South. Hanoi, Sa Pa, Ha Long Bay, Tam Coc, Hue, Da Nang, Hoi An, Nha Trang, Da Lat, Buon Ma Thiot & The Central Highlands, Mui Ne, Saigon, Me Kong Delta.
Fantastic countryside, people, food, experience. Fantastic. Just wish I could have kept rolling and done Cambodia and Laos with the people I met. Stupid job and responsibilities.
Hanoi was a great base to do and see so many things and is far more traditional than anywhere else. Hired a bike and rode the coastal rode (a la Top Gear) from Hue to Hoi An. Full Moon Party on a junk boat in Ha Long Bay. Climbed and trekked Mount Fansipan and the valleys around Sa Pa. Chilled on the beach at Hoi An and Nha Trang. Took a 3 day Easy Rider motorbike tour from Dalat to Mui Ne and stayed in remote villages and homestays. I got to see the real rural side of a wonderful country now thriving after all of the turmoil it has suffered.
Mui Ne was a little disappointing (basically a Russian holiday resort town). Saigon was mental. Hanoi on crack. Scooter drag racing at night. Great party scene. Cu Chi tunnels. Firing an AK. Me Kong Delta. Superb.
Get it booked. If you only have 1 week I'd stay in the North (Hanoi as a base) as there seems to be more to do and see (Sapa, HLB, Tam Coc).
If you just want a week chillin then Hoi An. Nice bike ride to the beach from the town. The town has a Mediterranean feel and is quiet and you can get bespoke suits/dresses/shoes made. You could also see the coastal rode (Hai San Pass?), Da Nang and Hue on day trips as well as the Vietnamese version of Angkor Watt.