1. Take the high-speed train to Guangzhou: There are many high-speed trains from Nanning to Guangzhou, and take the high-speed train to Guangzhou South Station. From Guangzhou South Railway Station, you can take the Guangzhou-Shenzhen-Hong Kong high-speed train to Shenzhen, and then take a long-distance bus or taxi to Shenzhen Bay Port in Shenzhen. After going through the exit formalities at Shenzhen Bay Port, you can walk to the registration hall in Vietnam and then go through the entry formalities. Once you have completed the entry formalities, you can take Vietnamese transportation to your destination in Vietnam.
2. Take the coach directly: Nanning has many long-distance buses to major cities in Vietnam, such as Hanoi and Mang Street. You can go to Nanning coach station, buy a ticket and get on the bus, and then go through the entry and exit formalities at the Sino-Vietnamese border port.
Please note that the above is the information I have learned so far. The specific itinerary and the choice of transportation may be affected by the timetable and policies. It is suggested that you inquire about relevant information in advance and consult the local travel agency or transportation department in order to obtain the most accurate travel plan.