No idea, but personally I would try it out. There are a lot of Cambodian and Lao vehicles in VN, so obv it is possible to bring in a vehicle without paying VNs punitive import tax, if the vehicle is going out again. The driving licence, that may be a problem since you need an A2 VN licence and these are very, very hard to get.
If you will be in HCMC then a large bike is wasted. It would be beneficial outside HCMC as going long distances on a 125cc, which is what you will get otherwise, in painful.