Take the KCR train no.T804 from HK Kowloon terminal 10.42 to Foshan arr. 13.22. You don't need to stop in Guangzhou (it's huge and ugly.) Spend a while in Foshan- look around the fascinating Zhumiau temple.From Foshan, you could take train K365 at 14.38 but it gets to Nanning at 02.24, so ride K483 dep. Foshan 21.37, arrive Nanning 0938. Plenty of buses go from Nanning to Hanoi, or spend the day in Nanning, which has beautiful ladies and friendly people, and take train T871 out of Nanning at 18.15, sleep on the train and arrive Hanoi 07.00 next day.(After a day in Hanoi, you'll probably want to go back to China.) Have a nice trip.