Make sure you head off to Sapa and surrounds for hill tribes, great weekend markets (there is an awesome market an hour or two from Sapa-its a day trip from most of the tour agencies so easy to find the name once your there)
Also Halong Bay is a must see. Avoid the cheapie boats. One sank a year or two ago killing everybody on board from memory.
We were not on bikes so can't give advice as to roads etc.
Can't wait to get back there on a bike. Whatever you do unless your hell bent on doing it or very mechanical avoid the Minsk's. Rent a Honda wave, it will do up to 130km(depending on load of course) and everybody can work on it if you have trouble.
I just had a 2 week trip in Laos on a Honda xr 250. Had lots of trouble with the bike and nobody would even attempt to look at it. It might as well have been a space shuttle to the average scooter mechanic.