Shenzhen (preferably Lo Wu) to Hanoi by long distance bus or train

I want to travel to Hanoi by either bus/train from Hong Kong on 30th Jan 2014 (the lunar new year's eve).

Where can I get bus tickets for SZ to Hanoi?
If there is no direct bus, I don't mind doing the SZ-Nanning-Hanoi route.

And if buses are too troublesome, I don't mind doing the train route (Lo Wu-Nanning-Gia Lam) but there is an issue. I know I can buy the Lo Wu to Nanning train ticket in advance at the Lo Wu station as it's only nearby Hong Kong but I have no idea how to get the train ticket for Nanning to Gia Lam in advance. I don't want to be stucked in Nanning with no tickets to Hanoi on the day I arrive Nanning.

Please advise..Thanks.