I haven't been since 1997 when I spent a month there. I LOVE Vietnam. I didn't get to Cambodia (visa problems), but I did fly to Bangkok and just make my plans there. It took me two nights in Bangkok to get to Vietnam (flight availability). The flights were much cheaper than setting them up here in the US (back in 1997).

found this itinerary on wikitravel a few months ago. 

Bangkok to Ho Chi Minh City overland...

http://wikitravel.org/en/Bangkok_to_Ho_Chi_Minh_City_overland

OK. My info may be a bit out of date, as i was there in 2001.. but here goes:

fights in and out of bangkok were very cheaper and generally pretty easy to arrange. as for bangkok itself... if you have never been there, it is worth spending a couple days to check out the major sights. personally, after 5 or 6 Bangkok visits, i can't stand the place (you seem to end up going through there to "take care of business"  constantly when you are in SE Asia). The heat and humidity and people and POLLUTION kill me. But the first time I was there, I enjoyed it. Check out the Royal Palace.

Siem Riep (for Angkor Wat)... used to be you paid for 2 days and got three.  I would say you need at least three days to see everything there without running around like a chicken with its head cut off.  Highlights = Angkor Thom (that's the one that they have not restored... a controversial decision, but it is really fascinating to see how the jungle can just take over...) and the one with all of the faces (sorry, drawing a blank on the name right now... you will inevitably get there). Also, there is a group of temples that is a bit farther away, where not too many people really go. We went, and quite enjoyed it. Fewer tourists, and a slightly different style of architecture. Most people hired a local with motorcycle to take them around to the different temples. I was with a friend, and we managed to hire a guy who had a little two-wheel chariot kind of thing that he pulled with his motorcycle. Not as fast, but more comfortable, and kind of fun. When we went to the temples that were a bit farther away, our driver took us the back way so we could see his village. We were apparently quite a novelty, because as we drove along, every person, young and old, would stop what they were doing, run to the side of the road and smile and wave to us. We felt like royalty.  But I digress...

I also spent some time in Phnom Penh, which I quite enjoyed. Stayed at one of those little guest houses on the lake. The Toul Sleng museum (a former high school that became the Khmer Rouge's Prison 21 torture facility) made for a very distubing but powerful experience. Our tour guide's family had been sent out to the country by the Khmer Rouge, and she really brought a powerful personal element to the museum experience. Since it was not so long ago, many of the older people in Cambodia have very vivid memories of the Khmer Rouge, but they are very hesitant to talk about it.

Also went to Sihanoukville, down on the coast... which was a beautiful little beach town, and ridiculously cheap (in case you want a few days of beach relaxation). Not so many tourists, which is always nice.

Never made it to Vietnam. Before my trip, I had heard from a friend that it was his favorite country in the area... but by the time I got there, the buzz among the travelers was not so positive. So I went to Laos instead, which was terrific.. but that's another story.

Good luck!

We are going to do the same trip over Christmas and New Year's.  When I get organized (I just got back from India) I will post my itinerary.  And certainly I will write a trip report.  Hopefully that helps, and it should be up-to-date.  There will be six of us traveling together.  Me, my wife, 19 year old daughter, 16 year old son, a girl friend of my wife's and her 16 year old son.  We spend two nights in Saigon, then a swing through Hue/Da Nang/Hoi An, three nights in Hanoi and three nights in Siem Reap.  

 I've been told to avoid Phnom Penh, by someone who knows better.  And unfortunately no time to run up to Laos.  Finally, too cold in the winter for Sapa, but it might be wonderful in the spring. 

We did the internal flights through a contact in Vietnam.  It appears that we saved about half. 

sounds wonderful...

I want to travel there if have chance....

It has been incredibly busy and fulfilling trip to Vietnam.  We leave this afternoon for Cambodia.  The upshot about traveling in Vietnam is that I highly recommend it.  There are some considerations.  The cities are gritty and intense, but in a good way.  Walking through Hanoi is like taking a step back in time 50 years.  Cluttered, crowded, vendors everywhere, almost no rules.  The buildings are not high, about 3 stories typically.  But the street scene is a complete jumble.  The air is thick with smoke and smells.  People cooking on the sidewalks, cafes spilling into the streets, small women chasing you with baskets of fruit.  The weather in Hanoi has been overcast, so the grittiness is emphasized.  Another consideration is that the tourist trade is pretty well set up here.  This is exotic, but hardly on the edge.  Everything is pretty well organized, and plenty of Europeans, Aussies and Koreans touring the cities and the countryside.  Finally, don't expect the infrastructure to always work.  Roads and trains are poor quality, so things that you think would take one hour in the US, or three hours based on experienced judgment, can take up to six hours in reality. 

We spent two days in Saigon.  Saigon is an easy city for walking around.  The number one cultural difficulty is crossing the street, but once you get the hang of it, you feel pretty proud of yourself.  Saigon is, depending on your mood, thriving and pulsing, or chaotic and crowded.  The street scene here is actually quite a treat.  Food hawkers, small stalls, plenty of pedestrians and mopeds everywhere.  The traffic is a libertarian's delight.  Stop lights, lane markers, speed limits are all "suggestions."  Mopeds even run around on the sidewalks.  They are like a separate species. 

The people are very nice, and very assertive.  Of course, you don't beat out both the French and the US military forces by being pushovers.  Our hotel is just off the river, which is your basic, muddy third world river.  It is used more as a street and sewer than anything else.  All the major sites are an brisk walk out.  The restaurants are really good.  Quiet ambience, much like the Parisian cafes.  the food has been extremely good so far.  We will venture a bit deeper tomorrow and eat from street vendors. 

The major outing today was a trip to the Vietnam War Remnants museum.  The perspective was definitely that of the current regime.  However, I must say, they handled the story in a very even-handed manner, relatively free of propaganda.  The story was brutal, and they held no punches.  It was emotionally very draining to see all the relics, photos and individual stories.  At the end of one day in Saigon, I definitely recommend it to folks that like an adventure.  But you have to stay on your game, all the time.