Yes, the same sort of heroic nationalistic stories that each of our countries tell us. Each country having a rich and old history with many heroes striving to preserve that heritage for their children and grandchildren. Yes, Thai, Lao and many others tell of similar stories.
But stories do not always feed stomachs. Throughout history we often do what our leaders have told us to do, not what we want to do. Coming from Vietnam I thought you would have been familiar with that aspect of our region's culture. Our countries, the way we speak our languages, the cultures and many other things were formed by the elites who lead our countries. If the leaders do not want to lose their power and instead would rather keep a separate country then they will do it and come up with a reason, along with many heroic stories to tell us so as to justify what they have decided to do. This has been the case for so long and I am surprised you are not familiar with it. Democracies tend to go with what their population want, but in other places sometimes it is the elite ruling circle's will that often prevails.
I think you have underestimated the strength of Vietnam as a state. The population base and the depth of self-identity in Vietnam is strong enough to ensure its survival. It is a mini China. If the Europeans had not intervened, the Vietnamese would have continued their southward expansion and absorbed parts of Cambodia, leading to direct conflict with Siam. Vietnam since its independence in the 10th century, have always been expansionist, and it was due to the ability of this Confucianist state to successfully increase and manage its population versus that of its southern neighbours. Despite her size, Vietnam has already since the early days of its independence, the largest population of any of the states in mainland southeast Asia, larger than either Siam or Myanmar. Indeed China's attempts to thwart Vietnamese expansion against Chinese tributaries such as Champa and Cambodia have all failed in the long run. The Ming invasion from China was not the first to to fail, the Mongols failed before them, even after conquering the powerful Southern Song. Post-Ming China was to view Vietnam as an independent tributary with similar status as Korea.
If China were to continue its expansionist activities in the absence of European interference, Siam and Myanmar would be in greater danger than Vietnam, given they had thinner populations characteristic of the Hindu-Buddhist countries of southeast Asia. They would have been overwhelmed by Chinese colonization and immigration if annexed by a Chinese dynasty. Vietnam has past this vulnerable stage.