If you search the forums using the search bar at the top right you will find loads of info about riding in Vietnam, more then enough for your trip to Sapa.
You can rent motorcycles from 'voyage vietnam' and 'Mr Cuong' in the north-east of the city, I believe the shops are opposite each other. Very reasonable prices. I dont think you can take them accross the 17th parallel though. You used to be able to pick up a bike in the south and drop it off in the north but thats not possible these days.
Routes can be found at 'minsk club vietnam . com'. Your route is well ridden and has made its way into the lonely planet guide. Its on the country roads here that you may ant to think about hiring a dirt bike, but then again the minsk is a pretty sturdy beast and almost everyone knows how to fix them.
Theres a couple of chaps (pete123 and someone else I forget...) on here who have ridden extensively in vietnam if you need any more info.
If air travel was cheaper Id be coming with you lol
best of luck!