I came across a reference in Alexander Woodside’s Vietnam and the Chinese Model to some documents which were exchanged between the Nguyễn and Chakri courts. There of course is information about Siam-Vietnam relations in official chronicles of these two dynasties, but what is unique about the documents Woodside referred to is that they are the actual letters which were exchanged between the courts, whereas what appears in a work like the Đại Nam thực lục is a summary of these exchanges.
That said, these documents are a little more complex than that. What the Vietnamese preserved were some of the letters which Gia Long sent to Rama II, as well as the translations of documents which were sent from the Chakri court to Huế. These documents are in a mixture of classical Chinese and Nom, and are therefore translations of letters which were originally written in Thai. At least some of the original Thai letters should still be preserved in the National Library in Bangkok. Therefore, there is a great research topic here. Comparing the original Thai letters with 1) their classical Chinese/Nom translations and 2) what eventually was written in official chronicles in both Siam and Vietnam, should lead to some interesting insights.
In 1989 Michael Eiland wrote a dissertation on the topic of Siam-Vietnam relations called “Dragon and Elephant: Relations Between Việt Nam and Siam 1782-1847.” In this work, Eiland argued that the Siamese and Vietnamese had very different worldviews, and that their conflicts were in part due to their differing cultural perspectives.
I did not find this work convincing, and in fact, the more I think about it, the more I am coming to think that, besides practical issues of geopolitical power, the Siamese and Vietnamese may have come into conflict precisely because they shared similar worldviews. Both the Vietnamese and Siamese viewed “foreign relations” as hierarchical or unequal. One was either a vassal or a lord. They had no concept of equality. The problem for the Siamese and Vietnamese in the nineteenth century, however, is that in practical terms they basically were equals.
If you read the Đại Nam thực lục or the Praratchaphongsawadan, what you find is hierarchical terminology. When Vietnamese sources mention sending a mission to Bangkok, for instance, it is to “tặng” (贈) gifts, that is “to give from a superior to an inferior.” In the Thai chronicles, however, the Vietnamese come to “thawaai” (ถวาย) gifts, that is “to give from an inferior to a superior.”
So there is a contradiction here. Each side recorded information which depicted its own side as superior, however officially neither side was superior to the other. Further, if we are to believe the documents which were exchanged between the two courts, or at least the limited number preserved by the Vietnamese, it appears that when these two sides actually interacted, they did so without hierarchy, for the language in these letters is polite but not hierarchical. For example, the first pages of two letters below, one from Rama II to Gia Long and the other from Gia Long to Rama II, respectively, begin as follows:
The Siamese King writes to the Buddha King of Việt Nam (and then it contains an ornate phrase which essentially means “for him to read that. . .”)
暹羅王書達越南國佛王，陛前玉覽 . . .
The Vietnamese King reverently writes to the Buddha King of Siam (and then it contains an ornate phrase which essentially means “for him to read that. . .”)
越南國王肅書于暹羅國佛王，臺前青覽 . . .
Again, these documents are polite, but they do not express hierarchy. It is of course also interesting that the two rulers referred to their respective counterpart as a “Buddha King” (Phật Vương, 佛王). Perhaps this was a novel way to express equivalence of some sort.
In conclusion, there appears to be a difference between what actually transpired when Vietnamese and Siamese officially met with what each side later wrote about these meetings. This then is what makes me wonder if the “clash” between these two people was not because of differing worldviews, but because of similar ones. It would seem that at a practical level these two peoples could interact with each other, and to do so as equals. However, there was apparently a domestic need on both sides to be the superior in this relationship. This is a need which the Siamese and Vietnamese both shared, and it was of course impossible to achieve, as there can not be two superiors in a relationship between two sides. Perhaps then rather than it being the case that the differing worldviews of the Vietnamese and Siamese led to conflict, it was that the contradictions of this shared perception made it difficult for these two peoples to peacefully coexist.