Thailand is a new country established by local Siamese and Chinese settlers. At the height of military conquest, the Thai occupied Laos, Cambodia, parts of Burma and ...southern Vietnam.
How do you define new? Thai peoples did displace the Khmer Kingdoms but not aware of any deliberate policy by Chinese dynasties. In addition Vietnamese people were not native either.
Tai people in China are known as the Dai ethnic group and share linguistic ties with Zhuang ethnic group. The other Chinese who settled in Thailand were those who sought greener pastures. Some Thais have blurred lines with some groups of modern day Chinese, though there are other components to modern Thai peoples.
Dai ethnic group in China, mainly living in Xishuangbanna, Yunnan
Northern Chinese usually hold the polical power, while southern Chinese excel in economy. The friction Southern Chinese with Vietnam comes because Chinese foot soldiers are mostly southern Chinese commanded by northern high ranks. Southern Chinese are the ones that pay their lives in battles.
In the modern day, the political power more distributed, though more economically developed regions might have better chances for high positions due to access to better resources.
Southern China is a broad area, what constitutes as "Southern" might be different for each person or region. Within the South there are many distinctions. The region surrounding the lower Yangzte delta is commonly referred to as "Jiangnan" and had been the most economically vibrant region in China since the Song dynasty. Many people from Northern China moved south due to the wars in the central plains. With the migration, came the shift of economic gravity that occurred over centuries.
Another economic center in modern China is the Pearl River Delta. It is likely the most free market region in China and is the home to many cunning business people. This region's economic ascendancy had been a relative recent experience in China's history. It takes a certain kind of mentality to thrive in a cut throat business environment.
The idea that Northerners hold political and military power came from the history of China. Most of the population was concentrated in the North for the formative years of China as a civilization, same with economic, military, and political power. The North tended to be the focal point of major wars (3 Kingdoms war, the world's bloodiest ancient war occurred in the central plains which killed 36-40 million) and natural disasters especially regarding droughts and floods. A lot of this had to do with geography. This necessitated the development of a bureaucracy to unite the lands under one management system to coordinate the management of water resources to prevent floods and droughts. The military was the means to achieve unification and push to geographic boundaries to enable a stable economy. If disunity occurred, war will happen.
https://en.wikipedia.org/wiki/List_of_wars_by_death_toll
https://en.wikipedia.org/wiki/List_of_natural_disasters_by_death_toll
Ancient China had a long list of wars, mostly occurring in the north.
https://en.wikipedia.org/wiki/List_of_Chinese_wars_and_battles
Western Han Dynasty population density map.
By the Tang dynasty, a greater proportion of the population were living in Sichuan and Jiangnan. Trade was supported by the Grand Canal built during the previous Sui Dynasty to connect the central plains with Jiangnan.
Out of historical experiences, Northern China tends to value military and political power/management since it was readily apparent it is needed for survival. At times when one was bad at it then millions would die (which happened) and there are few mountains or valleys to hide in, it is a wide open plain with multiple points of access that opens up to difficult to control lands (needing constant military and political attention) across the entire northern arc. Sometimes more than half of the population would die in a prolonged war. As a result of the wars from 184 AD to 217 AD during the collapse of Han dynasty the population was reduced from 65 million to 15 million.
The Jiangnan region since the last few centuries had the highest concentration of China's intellectual base. A disproportionately high number of those responsible for major scientific and technology breakthroughs, including other intellectual areas were from this region, along with industrialists whom I respect. People there value education, commerce, have an open and flexible mind, the drive to succeed at all costs, the region was a magnet for talent, and had a relatively stable environment to operate in. This creates the environment we see today where China's top leadership tends to be from Northern and Eastern China.
Vietnamese usually don’t have any problem with Northern Chinese in daily life.
In contrast Vietnamese consider southern Chinese such as Hainanese and Cantonese as dirty and greedy.
Maybe due to distance and isolation, northern Chinese generally don't have much thoughts on or know much about Vietnam or Vietnamese. It is seen as just another country to the South. People there focus on Korea and Japan.
Generally speaking, Northern Chinese aren't as apt to business as those from the coastal South. The culture is different regarding money. People are pretty much a product of their environment.
If we didn't win land war against Vietnam, how possible Vietnam become a vassal state of China for millennium? 
, keep delude youself more
