Southbound, take an overnight sleeper bus (a bus with sleeping-berths) from Kunming to Hekou on the Vietnamese frontier. There's also an earlier departure, and there maybe later ones. The bus fare is about 87 RMB. At Hekou, cross over the border to the Vietnamese side and use a taxi to get to Lao Cai 3km from the border post. Then use an overnight train from Lao Cai to Hanoi. You may want stop off at Lao Cai to visit Sapa, 40km away, before going on to Hanoi.