Vietnam is fortunately a thin shaped country that makes it easy to travel without backtracking too much. And as you progress from north
(or vice-versa), you would be able to perceptively notice the changes in the culture
, as well as have a historical appreciation of the southwards direction of the Viet Cong
One week trip
You cannot do Vietnam any justice with this short period of time. Yet if you are hurried, just spend three days in Ho Chi Minh City, fly to Hanoi on the fourth and open jaw out of Vietnam on the seventh. In one day each you should be able to see the major sights of both cities, returning exhausted to your hotel room. Visit the Reunification Palace, Cholon, Dong Khoi street and the 'American War' museum in Ho Chi Minh City. Spend the next two days on a trip around Ho Chi Minh City that includes the Caodai temple, the Cu Chi tunnels and a part of the green Mekong around Vinh Long, Long Xuynh or Can Tho. Fly to Hanoi, and check out the sites I have listed in my Hanoi node. Do an overnight trip to see the natural limestone formations of Halong Bay, and maybe visit the Perfume Pagoda on your return to the capital.
Two week trip
As above, but treat yourself to an extra day to each of the cities. Also instead of flying out of Ho Chi Minh City, make your way up overland to Nha Trang and then fly to Hanoi. En route to Nha Trang take either the highland or the lowland route - if you are into hills stopover in Dalat, a mountainous resort with lots of French Alpine ambiance. Or if you like the check out Mui Ne (scene of some awesome sand dunes adjacent to a nice tropical resort without too much development so far), and Phan Thiet (site of some historical Cham monuments). It is probably best that you rely on buses if you want to stop off along the way, otherwise go directly by train. Nha Trang is a nice place to relax, swim and stuff yourself stupid with crab and beer (despite what others might say, it is not really a diver's paradise).
Three week trip
Four days each in Ho Chi Minh City and Hanoi (there is little reason to spend longer in either place), plus the aforementioned side trips. This time take the train to Danang. Give the town a miss, and head to the beautiful Japanese/Portugese trading post of Hoi An for a day or two. Then drive up to Hue, up and along the chunderous mountain passes that overlook China Beach. Hue has a number of historical attractions, including the Imperial Palace. Further inland in the mountains is Khe Sanh and other battlegrounds around the former demilitarised zone. Fly or take the train up to Hanoi.
Four week trip (or longer)spend longer in the Mekong delta
As per the three week trip. But depending on the amount of time you have you may wish to try some further excursions:
visit both Dalat and the attractions on the low land route
travel up to the mountainous villages around Sapa near the Chinese border, home to many ethnic minorities.
go even further west from Sapa to Dien Bien Phu (a round journey overland would take about five days)
spend longer in the demilitarised zone; closer to Hoi An is the unfortunate village of Son My, better known as My Lai.
backtrack down the Ho Chi Minh Trail, via the Central Highland regions of Kontum and Dak Lak.
Where not to visitVinh - although the poorest part of Vietnam could well do with your tourist dollars.
Haiphong - just contains a harbour and smelters, and Vietnam's only (legal) casino.
Ca Mau - You will never see bigger mosquitos anywhere else.
The Spratly Islands - Vietnam has started tours to this reef in the South China Sea. Unfortunately six other countries will think you are invading their territory.
Saigon Water Park
Vung Tau - Was a place for R&R, now it supports Vietnam's offshore oil industry.
off a minecleared path in the DMZ