This map is a rough approximation of the borders during 1400 CE or near the time of the formation of the nation of Lan Xang, that many claim is 1353 CE or 1896 by the Lao calendar.
There are conflicting dates and histories written by each of the societies involved, of course. So, when writing around this era, I would definitely exercise caution and cross-check as many facts as you can, taking into account the bias of different historians.
These borders will change and flux over time, and between 1707-1893 CE, a contentious period emerged when Lan Xang was split into the realms of Luang Phrabang, Viang Chan, Champasak,and Muang Phuan.
Given that there are at least 160 ethnic groups speaking over 82 distinct living languages or combinations thereof in modern Laos, the situation was likely very similar during the ancient era, with possibly more. Undoubtedly there may have been some additional societies who were not well documented or lived only briefly in the region.
As in modern times, near any nation's borders, travel and regular people's sense of who they are and their particular loyalties would likely have been very fluid. Villages would come and go, some would get overgrown, others destroyed by the modern era, wiped out without a trace during both major and minor conflicts.
To that end, for writers of fiction, I would consider it to be a time of either great possibilities or one that could be very intimidating to tackle. How would you approach it?