Firstly, there was no such thing as Laos, Thailand, Vietnam or Cambodia two hundred years ago. These states were all created in the 20th Century.
Secondly, the kingdoms that existed before these states were created were no based around land area or national borders, they were based on the prestige of the king (which was measured by how many people he ruled over and how many slaves he had) so it makes no sense to draw maps showing different land areas.
In addition, many communities actually pledged their loyalty to more than one king, so it was common for people in Isarn, to support both lan xang and siam when it suited them.
Finally, the kings themselves did not represent seperate blood lines (ie, that some people might want to equate with national identity) because they took their wives from neighbouring kingdoms in the hope of expanding their influence.
The problem is that most of our education systems todaay teach us that the modern states of Laos, Thailand, Cambodia, and Vietnam are the direct descendents of the pre-modern kingdoms. That is why we spend all of this time arguing about which land and which people belonging to which state. But it is just not the case that a pre-modern king from any of these kingdoms would identify with the modern state. Fa Ngoum was educated in Siam and married a Khmer princess. He would turn pale at the though of a kingdom ruled by a pro-Vietnamese communist party.
The best thing we can do is open our minds to the idea of many histories and the idea that all histories are contested and are written by people with specific interests. The sooner we realise that the most important aspect of our identity is that we are all 'human' rather than 'national', the better things will be.