I dont think you can get into Vietnam with any other than Vietnam-registered bike, and even then there´s a MAYBE. Saw a couple, in Laos, who had bought Minsks in Vietnam, and crossed into Laos with them, they planned to take them back, but I dont know if they did.
Some even seem to make up stories about how theyve done it while actually they havent, thats how complicated it is. Theres been some rumour about things changing slowly, and I hope they do, but so far there hasnt been any confirmation.
The "under 175cc rule" (if there ever was one, or if it was just someones wishful thinking), seems to mean nothing, either. It is a big pain-in-the-butt to get in there with your own vehicle.