So we did the trip in 2013 :) It was so great - Vietnam, Lao, Cambodia on the same bike. We could buy motorbikes in Ho Chi Minh City.

We had a lot of 1$ bills...It worked !!!

Good news

Put some photos up on the site and tell use the story bier

Wayne

I put some photos, teasers here:

CRÔSS MÊKƠNG PÂĐĂKA RĂLLY 2013 (Vietnam, Cambodia, Laos)
http://www.horizonsunlimited.com/hub...s-m-k-ng-69032

As many said before: limit to one country if you only have a couple of weeks.
The only sure way to cross borders is to have a local bike, registered on your name. If you buy in Vietnam, no problem taking it into the other countries. If you buy in Cambodia, no problem to take it to the other countries, except Vietnam.
In Cambodia, buying is easy, but registering can take time, at least one month, sometimes up to three months before you get the license plate, which you need to cross the borders. A local scooter/bike is available (new) for $1300-2500, depending on the model.
In Vietnam, I think you can buy a second hand bike, and have it registered pretty fast. But I have no experience with it (as I'm living in Cambodia).
But, if it's only for a few weeks, just stay local, rent a bike, and tour around. Take in the next country on an other trip.

After getting advice From GTriders then I hired in Chaing Mai and rode into Laos no problem. All the Advice on that site
Safe riding
Paul