We travelled from Hanoi to the north, sidetripped to cambodia and laos and worked our way up to hanoi from Ho Chi Minh.
We bought a minsk off Coung in Hanoi. 450 us gave us a thoroughly refurbished and reliable machine and an excellent service. in 10.000k across the indochinas we had only one breakdown. We sold it for 350. So you may want to get one off him. He rents Minsks for about 6 usd a day. His brother in Ho Chi Minh can return them. He also rents out Baja 250s.
Generally the roads are very good in vietnam. Get the Blue roadbook with the white bridge from the bookstores near the Hanoi post office. It's getting slightly outdated, but still an invaluable help. Also check www.gt-rider.com
for upToDate road info.
I can passionately recommend you to go via the ho Chi Minh highway. You're on your own through pristine jungle on a narrow winding concrete road with few but very friendly villages along. Highway 1 is ugly and dangerous. And full of tourists. The only thing is: officials may turn you away when the road is blocked by landslides...
In any case, when in Hanoi, pass by the Highway 4 restaurant in the old quater. Digby or Glenn (exploreindochina.com) are there regularly and can give you heaps of advise.
No helmet laws, but the ubiquitous passing chicken or pig may have you want one.
Insurance is theory. Ask for the papers when buying or renting a bike, there's 3 of them, about bank card size. One of them is the insurance. You'll only need them te resell your bike.
feel free to check our pics. Ther's a few sets on vietnam here:
you don't stop playing because you get old. You get old because you stop playing