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.
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