As has been stated if you want to visit all of these countries with one bike you will need to start in Vietnam and you will have to buy one to take it out of the country.
As for the time taken nearer three months than three weeks to do this trip justice, at least six weeks to visit all four and that would be rushing. I would recommend you stick with one or perhaps two countries. If you have not been to SE Asia before I would suggest Thailand as a good place to start with plenty of dirt roads in the north to explore.
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