I will arrive at nanning train station and want to take a bus to Ping Xiang border to Vietnam.

There is no bus from the train station. Several long-distance buses at Langdong Bus Station that can take you to Pingxiang directly. The schedules are from 07:30 to 20:30 with the interval of 15 to 30 mins.
Or you can take the train from the railway station. There are three trains that can take you to the destination. It will take 5 to 6 hours on the way.