Ok I admit my info on Vietnam could be outdated. Once went with friends to try it on 115cc scooters (Thai plates) from Cambo & Laos, and failed to get in. We weren't prepared to pay a lot of 'baksheesh' for entry, though. At that time (2006) there was a persistent rumour, that under 175cc is okay, that's why we went with those scooters. I thought language barrier at their borders was substantial..