As far as I know, there is no direct train from Guangzhou to Hanoi. Nanning can be used as a connection point. You may take a train from Guangzhou to Nanning first. Upon arrival in Nanning, both coach and train to Hanoi are available.
The coach departs at 8:00 / 8:30 / 10:00 respectively per day. The coach ticket costs around RMB170.00 net per person. The international passenger train departs at 18:15 and arrives Hanoi at next 7:00. The hard sleeper train ticket costs RMB181.00 and it is RMB290.00 for soft sleeper.
Good luck!