i will be going to vietnam on may.

i want to go to guangzhou from hanoi by train and then exit from hongkong. I know a visa is not needed for 2 days stay in hongkong, do i need to get a visa for 2 days stay in GZ? if yes, is it better to get the visa here before leaving or is it easier to go through a travel agency in hanoi, or do you have any suggestion about it? thanks