I lived in Kunming, China for a couple years in the late 90s and twice I took the narrow gauge French railroad from Kunming to Sapa at the border of Vietnam. The mountains around Sapa are absolutely amazing, you can find daily guides that will take you on hikes up into the small villages and, yes even some of the poppy fields that are up there. From there I took the rail down to Hanoi. You definitely need to go to Ha Long bay with it islands, karsks, and caves. Locals will ride up alongside the main boat and you can buy fresh seafood that the crew will steam or boil for you, they usually seem to have beer on board as well, amazing scenery in Ha Long bay. Hanoi itself is a beautiful city with amazing French architecture, alleys, small streets, coffee shops, and markets, if you get to Hanoi make sure you checkout sword-restored Lake in the middle of the city too. Heading south from there there's not much to see until you get to Hue, amazing temples around there with river tours that will take you from temple to temple, and the DMZ is not far from here either, pretty incredible tours around there to take with a interesting history to study of the war, I was actually able to find bullet shells and even 3 inch shells in the dirt around some of the areas. Da Nang is nothing special, I liked it because the girlfriend I traveled with one time had family that was from there and we hung out in the city during Tet (the new year) which was really cool, but south of there is Hoi An which is a great place to check out, small little town with some nice architecture and neat markets in it. South from here is the beach resort city of Nha Trang, nice place, the second time I went to Vietnam the girlfriend I was with that time her family is from there so we hung out in the city for a little while and I enjoyed myself, really good food as well, excellent beaches. Nha Trang is a good jumping off point to get up to Da Lat a resort area up in the mountains and another really cool place, little bit out-of-the-way but if you got plenty of time it's not to be missed. Then from there you go to Ho Chi Minh City. Ho Chi Minh City is nothing but a giant city but it's kind of neat, watch out for all the scooters. If you can get out of Ho Chi Minh City get yourself out to the Mekong delta. In some of the delta port towns find the open air markets where you can buy huge prawns I mean literally the size of lobsters, buy them cheaply in the market and pretty much any restaurant, as long as you buy beer or drink, from them, will steam them up for you, just terrific amazing experience.
So there you go, I was there twice traveled up and down the whole country, both times with a native speaker, one a French-Vietnamese girl, the second time an American Vietnamese girl, my experience was pretty incredible both times visiting their families and having a native speaker with me but the country is just beautiful, the people are amazingly friendly, I really enjoyed myself. The food in Vietnam, in my opinion from somebody who's traveled all over Asia a number of times, is the absolute best
I kind of describe things opposite from what you'll do since I started to the north, so for you south to north it's: Ho Chi Minh city, then the Mekong delta, Dalat, Nha Trang, Hoi An, Da Nang, Hue, Ha Long Bay, Hanoi and then if you can get up to Sapa and explore the mountains up there, definitely worth it.
Hmong children playing Sapa - BW by rich8155 (Richard Sintchak), on Flickr