Travel

Go from Phu quoc to Sg by air, sg to hanoi by air then go by train from Hanoi to Lao Cai. Train costs a lot more than bus (cheapest for train is 400k and if you are not as small as an average Vietnamese I doubt you would fit (400k is for the top bed of a triple-bed in a six bed cabin). Also take more time than bus and you still have to take a bus from Lao Cai city to Sa Pa which is the most tricky path with bad roads and accidents (40km from Lao Cai city to Sa Pa takes around 1 hour by car). I used to work there, each time I go back and forth between sapa and hanoi I take the day bus, much safer to travel during daytime. If you don't have much time then go by night train.

As for where to stay, it depends, if you don't mind some inconvenience you can book a hotel room in sa pa town for one night then see if you want to stay at a local village house. The corlorful market is not near sa pa but Lao Cai city, maybe you can go there before you leave Lao Cai to go back to Hanoi. Aug would be a good time to be there for me, not really a tourist season so the price is not too high and of course not too crowded.