Hi,
entering Vietnam with a foreign bike is some kind ofa lotterie. There is a possibility to apply for a permit, a termporary import of a foreign motorcycle, and this permit must be send to the Vietnamese ministry of trafic and transportation. Or similar. They might or might not issue the needed documents but regardless whether the permit will be issued or not the fee must be paid. As far as I know we are talking about 1000,-US$.
There is a travel agent, headquarter in Hanoi, and they promise to get a permit for you. The price is roughly 500,- US$. BUT this agent is extremely moody, slow, the hell knows what but anything else than relaible. I tried for more than a year to get hold of this agent, travelled to the office in Ho Chi Minh City, called, send email but I never got any respond.
I tried the version with the little group, got three gusy together, those guys tried to contact the agent but also it was for nothing.
When finally after about at least one year I got a feedback it was all for sudden possible to entre Vietnam with a motorcycle as a single rider.
Did I mentioned to name of the agent? Just in case it was Flamingo Travel.
Entering Vietnam without permit I did as well. But, agian another but, I can not really suggest this. You are illigal in the country, if you get pulled over by the police and they might confiscate the motorcycle. The Vietnamese custom will not let you go, not for money and nice words. You have to smuggle the motorcycle. I did that and I was extremely careful always got both eyes on the road as in Vietnam got a lot of traffic control points and those police officers like to stop motorcycle. Actually they only stop motorcycle. So I was there with my Triumph Explorer 1200 for one month and most of the tiem I hide the motorcycle in the garage. Coutnryside it might work or not but in the cities you can be sure that the motorcycle is gone once you get pulled over.
A final word: the situation might change fast, speically in Vietnam. So what might be valid today might not be valid tomorrow anymore.