My wife and I visited Hanoi, Sapa and Halong Bay last year and used a travel agent (and house/room rental B&B) through AirBnB. It was the many positive reviews, pictures on Facebook, etc., that provided confidence. The people whose house we stayed in (Hanoi) and helped us with all the bookings are Dong (Vietnamese) and Narelle (Australian) of Dong's Travel https://www.airbnb.com/rooms/255694 They are both lovely people and can help out a lot.