Isn't Dong Dang and Huu Nghi the same border crossing? There should only be one international crossing point there - it's the same one whether you arrive on foot or by train although there should be a dedicated channel for train travelers to use.
On the Chinese side, the border crossing is called "You Yi Guan" 友谊关 or Friendship pass and yes, buses from Nanning would go there. I'm not clear on whether there are through buses (like on Laos-China, Laos-Thailand and Laos-Vietnam routes) or whether a change of bus takes place at the border. Most likely it's the latter (as I've never seen Chinese plated buses operating inside Vietnamese borders) and if you book a ticket to Hanoi, you will probably be put on a different bus once on the Vietnamese side of the border once you show your ticket.