yes, there are express bus from naning to Hanoi. Turn left from Nanning railway Station Exit and walk a few meters, you will find a ticket office for the bus to Hanoi. if you can not find it, you can just take a taxi to Langdong bus station (琅东客运站).
It will take 7-8 hours from Nanning to Hanoi. There are trains available, it takes much more time and passengers have to get off train for two times while pass the border. Bus are more easy to get. You can ask for the train information when you reach Nanjing, but I still recommed going by bus.