There is an extremely important text for the conflict in the 1830s between “Vietnam,” “Siam” and “Cambodia” that I have never seen an historian use before, and that is the The Strategy for Pacifying the Siamese Raiders and Thuận Bandits (Tiễu bình Tiêm khấu Thuận phỉ phương lược 勦平暹寇順匪方略).
This text contains very detailed information about the conflict between “Vietnam,” “Siam” and “Cambodia” in the early 1830s; much more detailed information than the Nguyễn Dynasty chronicles, The Veritable Records of Đại Nam (Đại Nam thực lục 大南寔錄), contains.
And from those details, one can gain very interesting insights into that conflict.

There is a document in this text, for instance, which talks about driving some Siamese forces out of Cambodia in 1834. It talks about how two groups of troops were involved in this effort: one Vietnamese, directed by Nguyễn Dynasty military officers, and the other, Cambodian, led by some oknha, or Cambodian officials.
As these two groups pursued the Siamese, they passed through areas where the Siamese troops had camped, and they had all been burned to the ground.
Eventually a Nguyễn Dynasty officer captured and interrogated a couple of Cambodian soldiers who had been fighting with the Siamese.
These two men revealed that the Siamese forces were actually made up of some Siamese phraya, or officials, as well as a Cambodian oknha, and that the troops consisted of Siamese, Cambodian and Lao soldiers. (8/8b-9a)

In other words, in this conflict you had Nguyễn Dynasty officials and Cambodian oknha fighting against Siamese phraya and Cambodian oknha who had Siamese, Cambodian and Lao troops. . .
So. . . who exactly was fighting whom? It certainly wasn’t a conflict between “the Vietnamese,” “the Siamese” and “the Cambodians.”