News on the grapevine is that travel on German trains from Sunday 9th December 2018 , and the next 6 months, will be available for booking from 16th October.
If you want to travel between Sapa or the Vietnam–China border and the Vietnamese capital, Hanoi, taking the Lào Cai–Hanoi train is the best option. On my blog, I show what the journey is like and how you can book tickets.