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).