I asked that same question when I first joined and got no response because it's been discussed so many times. Short answer is, if you want a dirt bike 250cc or bigger your best bet is to buy it in Thailand. You'll be able to drive it throughout SE Asia but NO GO for Vietnam. Buy a cheap one there or rent. Your other option and the best in SE Asia, IMHO, is to buy a smaller 110-125cc in Vietnam ($400-$700) and drive it where you please. That's what I'm doing in October. Hoping to find a Suzuki GN125 in Han Oi, throw some 50/50s and rack on it and take off, ending in Malaysia. If I get the urge to do some serious off roading, I'll just rent a big bike for a week in Cambodia. SEARCH this site and Horizons Unlimited, there are many people who have done and are doing the same thing. Good luck and safe trip!