Ignore my reply if your planned trip is part of a bigger one, and you're on your way on a big bike.
What we did/ Bought a Minsk 125cc 2stroke in Hanoi, drove it around North Vietnam, laos, Cambodia and back into vietnam to hanoi. Had not a single problem with crossing borders. The bike only failed on us once (burnt generator in Cambodia, BIG problem, but it got fixed in a few days). Cambodian roads were very hard on the bike, but nothing stops you to park your Minsk in Phnom Penh and rent a proper dirt bike (Baja or xr 250) for a week or so. Alltogether we were rather unprepared -we learned how to ride a motorbike in the old town of Hanoi-
Fantastic trip, 10.000km, 3 months loved every single one of them. Don't let people put you down on Vietnam, it is a morotbiking heaven. People who didn't like vietnam didn't try to get off the tourist trail. E.g. riding the Ho Chi Minh highway all the way was an incredible experience, not in the least because of the super friendly people we met.
If you venture into Cambodia, get the Adventure Cambodia book, by far the most challenging AND rewarding book ever, also log on to the www.GT-rider.com
website, they can provide you with the best, most accurate info and the only reliable map of Laos.
let me know if you've more questions
you don't stop playing because you get old. You get old because you stop playing