I've just left Vietnam after a month of nonsense. We eventually got our bikes into the country(TT600r's) but required a letter from the prime minister, a heap of cash and a mound of paperwork to do this.
While we were waiting for the papers to get sorted we hired some Minsks and went upto Sapa in the northwest for five days of incredible views and mountain roads.
December is the best time of year to be there.
You can't get anything bigger than 175cc's into Vietnam! I thought DT175s would be the best bikes to get there, but i only saw 1 in the whole country and i think that was imported by an ex-pat.
You can buy a Minsk for $500US brand new, ride it through the country and then sell it for $350 in Saigon. The other option is to hire($5 a day, including spares and some tools), we got ours from Nguyen Hung in Hanoi(0084 9351282, 0084 912214554(mob), 162 Tranh Quang Khai Str. in the old quarter)
Apparently Nguyen's brother does the same thing in Saigon so you can hire in Hanoi and ride all the way down to Saigon and dump it with him. Check it out with him though.
If you contact Mr.Dan at Highway4 restaurant/Explore Indochina (www.highway4.com
he could give you some other good contacts and a route.
Everyone in the country seems to know how to fix a Minsk. The bearing on the finally drive of mine collapsed, a guy across the street saw me and within an hour had it fixed, he charged me $6 and apparently that was a rip-off!
Check out our weblog at http://moto-hike.blogspot.com
for some pickies of our Minsk ride(click on Vietnam1 in the galleries link on the left)
Cambodia is a different matter and once you get there you could buy a bigger bike with 'Asia plates' on and ride around Thailand and Laos as well. Try Jim at Hotel California, Phnom Penh for better info on the situation here.
Have a good one......