Haikou via bus/air to Hanoi

If I take a bus HK to Hanoi via Nanning, is there a place to get a visa? Would I be better off getting a visa on arrival and flying from HK to Hanoi? Would it be possible to fly to Hanoi and return by bus? I would like to experience the bus. I don't know how many hours it would take, but I would be able to view the countryside. Amer passport, living in China