You would better contact some international travel agencies to arrange the tickets or purchase them at railway station. Usually, the international train tickets could be booked around 40 days in advance.

IM not sure train from Nanning to Hanoi you can book online ,
but very easy buy train ticket from Nanning to Hanoi
everyday have a train from Nanning to Hanoi , no more people use this way , its expensive (~~~500RMB ~~~ 75$)
You can buy bus ticket , there's bus leave Nanning to Hanoi in morning , and evening arrive Hanoi , you can ask your hotel serve man , there's a lot sell Nanning-Hanoi train ticket & bus ticket!