Burma excluded, crossing from other countries into Vietnam is among the toughest in that region. They are known to reject all foreign bikes regardless of engine size (even the rumour that under 175cc=ok did not seem to be true, but my information is from Dec-2006, so may be outdated!)
Having a bike with Vietnamese plates may of course be a big help, but I´ve got no first-hand info on that. In Laos, we met some tourists with Minsks bought from Vietnam, and they´d been able to cross from Vietnam, were at least planning to return there from Cambodia, but I dont know if it worked or not.
Thailand, Cambodia & Laos should all be do-able with a bike registered in any of these countries.
We tried to enter Vietnam with Thai-plated 115cc ´chicken chasers´ from Cambodia & Laos in 2006, went to 4 international border, one of which we tried twice. No go, but we had a group of 8, I believe that was also a factor, maybe if we had been only 1-2 people we could have possibly made it. Nothing is written in stone over there.
A little baksheesh every now and then may help smooth things up sometimes, but I wouldnt plan on bribing my way through every border in that region.