I have done this run several years back. I hired my Minsk 125 from Flamingo in Hanoi. The ride took six days leaving Hanoi for Mai Chau for Homestay at a White Thai village. Day 2 was on to Son La...(you can turn off for Sapa from here). Day 3 to DBP for overnight. Day 4 Lai Chau via Sin Ho. Day 5 across the gate past Fansipan and onto Sapa. Rode bikes to Lao Cai to meet with Flamingo agent at train station, where bikes were cleaned, drained of fuel and sent back to Hanoi for $20 each. Bike rent in Hanoi was $22 for my bike. We then got mini bus back to Sapa. Left next day for Laos border crossing at DBP. The roads are all in great condition though Sin Ho was being built at the time (2013). Hung at Flamingo had the bikes well prepped and gave us hotel and guesthouse info for each town. There are always concerns on trust, but just go with your gut, most Viet are good people.....learn some of the language to be polite, you get treated different if they think you understand them.
See my report here....Hanoi to Sapa via Dien Bien Phu
Chuc May Mon....good luck.
Enjoy the view down to Mai Chau
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