Not sure if this will help but this is the advice of Flamingo Travel based in Hanoi (http://www.flamingotravel.com.vn). As far as I know, Flamingo will be able to help to buy the insurance.
It is a legal requirement to have a Vietnamese Motorbike insurance Insurance policy as well as a Vietnamese driving Licence, but most locals and tourists get by fine without. The Vietnamese traffic police tend to leave the tourists alone because they don't feel confident to deal with it. Vietnamese motorbike insurance costs around $15 for the year.
To make a claim it can be difficult and its not really how the Vietnamese deal with Accidents. Vietnam can be a very strange country with regard to how accidents are resolved. The bigger vehicle usually pays (i.e. car drivers pay motorcyclists, motorcyclists pay cyclists). However, if you have an accident, first you have to talk with the other party to find out who has to pay and how much. If you do not agree with each other, then the police will be called and they will do their job (usually takes time) and you have to stay in Vietnam until they have the official report. Vietnamese people usually expect you to pay, even if you are right. Do not be surprised and stay calm to solve the situation. If you purchased an insurance policy, and if so request you can request to the police to file a claim.
I didn't have insurance (or a Vietnamese licence) when I rode in Vietnam... and fortunately avoided some very close calls.
I think you may strike trouble getting the bike out of Vietnam. The bike I bought came with the registration papers of a previous owner.... not the guy I bought it from. I wouldn't have the first idea of how you'd go about getting it transferred into your name to get it across the many borders you'll have to cross. I think that's an important one to resolve. If you are intending to buy new, maybe a bike shop will know what's needed? You can almost guarantee that if you are buying second hand, they won't know.
I gave my bike to a charity when I left Vietnam... and brought my own bike back to Asia for the next trip. The charity weren't interested in the bike unless it had the registration papers, btw (which it did).