The boundaries that define area studies in academia (Southeast Asia, East Asia, South Asia, and so on) emerged during WWII. For the first time the US was fighting a global war. They needed lots of local information about lots of far off places. At Yale they began collecting and collating that information, dividing the globe into different regions based more on geopolitical concerns that any deep and thoughtful reflection. But after the war, academics tried to argue and emphasize the historical, social, and cultural identity of these regions (e.g. What is South Asian culture, and how is it distinct? etc.) These ideas have been contested. Today still, Harvard still considers Vietnam part of East Asia. More recently, a Dutch scholar Van Schendel wrote a fascinating article in 2002 that showed the limits of area studies by proposing that a counterfactual 'Zomia' region in the highlands that span from S.E. Asia into China and across to Afghanistan. He picked three upland locales, just a few dozen kilometers apart -- one in E. Asia, one in S. Asia, and one in S.E. Asia -- to show that they all had far more in common with each other than Beijing, Bangkok, or Bangalore, the lowland cultures in their respective regions. As you can see, the current national boundaries and regionalization often do not accurately capture either the peoples or cultures within them.

The people of Burma (though not the majority, just some major minority groups like the Shan), Laos, and Thailand have a common linguistic and cultural ancestry as Tai peoples that migrated out of what is today southern China in the 6th-8th centuries, and mixed with Khmer and Mon peoples. They developed in a different cultural sphere thereafter, forming part of Sukhothai kingdom in the 13th to 16th centuries, and then the Lan Xang kingdom. The divide with its eastern neighbors can be seen in the spread of Buddhism. Burma, Laos, and Thailand all are Theravada Buddhist countries, while China and Vietnam practice Mahayana. Laos and Cambodia were always linked to political, cultural, and religious trends in Siam. French colonial administrators were always worried about losing out to Siamese influence in those two areas, and Siam often tried to retake and incorporate border provinces within Indochina. A contemporary example: the Lao communists killed the last of the Lao monarchs, placing them in concentration camps after 1975. But the Lao people miss their monarch. Today they have taken to adoring the Thai royal family. Keep in mind as well that more ethnic Lao live in Thailand than live in Laos, which has almost 55% of its population comprised of upland ethnic 'minorities'.

Conversely, Chinese (and later Japanese) influence was predominant in Vietnam. Still in the early 20th century, most Vietnamese elites were learning about the West by reading Chinese translations of works in English or German. This goes back over a thousand years to the Qin dynasty, when Vietnam was incorporated as a Chinese province. Over the next 1400 years, the area that is today northern Vietnam -- the southern 2/3 were only occupied by Vietnamese or 'Kinh' peoples beginning in the 16th century and not really in force until the 18th -- was often controlled by Chinese kingdoms. As Keith Taylor has argued, Vietnamese history is characterized by long periods of cooperation between Chinese and Vietnamese political elites, and short ruptures. French colonialism seems, in retrospect, a fleeting period that in comparison to China, had little impact.

David Wyatt, Thailand: A Short History (Yale, 2003). Keith Taylor, A History of the Vietnamese (Cambridge, 2013).