Is it possible to transport by land (bus or train) from Guangzhou to Hanoi (Vietnam)?

There is no direct way, but you can still accomplish it with a transfer. Take the long-distance bus to Pingxiang, Guangxi first. As I know, only one is available daily at Yuexiunan Bus Station, leaving at 19:40. It costs CNY280 per person and takes around 10 hours. Then transfer to a train to Hanoi at Pingxiang.