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Alligator, your chronology is mixed up WRT Lan Xang and Vietnam. For Vietnam, this is more of a quibble, but by the French invasion Vietnam was called Dai Nam, not Dai Viet. As for the former, well, by 1858 it had been defunct for 151 years.
Lan Xang's last effective king was Souligna Vongsa, who reigned from 1637 to 1694. Following his death, there was great chaos (four kings between 1694 and 1698). Finally in 1698 the Trinh lords of northern Vietnam sent 9,000 troops and installed his nephew Chao Sai Ong Hue on the throne of Vientiane, capital of Lan Xang. But most Lao lords detested the Vietnamese and their proxy, Sai Ong Hue. In 1706 his cousin, Chao Kingkitsarat, gained an army from the king of the Sipsong Panna and seized the northern city of Luang Prabang. Then in 1707, either through the intervention of a Thai army or through voluntary agreement (the latter is more likely IMO, and is the account given by the
Luang Prabang Annals), Kingkitsarat was given the north and Sai Ong Hue was given the south. This ended the unified Kingdom of Lan Xang and created two separate kingdoms, the kingdom of Luang Prabang under Kingkitsarat and the kingdom of Vientiane under Sai Ong Hue. In 1713 the latter split into two again, with a kingdom of Vientiane and a kingdom of Champasak. So by 1715, Lan Xang was gone and three Lao kingdoms were in its place. And all three kingdoms were increasingly falling under Thai hegemony.
In fact, by the time of French conquest there were no independent Lao states. After 1767 (when Thailand became much more powerful following the political turmoil of the Burman invasion in the 1760s) Lao princes west of the Mekong were forced to abandon their ties to Vientiane and Champasak and be absorbed by Thailand, and millenarian uprisings among the Lao from 1791 to 1817 were brutally suppressed. In 1827, Anuvong, king of Vientiane, revolted and brought his army to a three days' distance from Bangkok, but his rising was crushed, Anuvong was tortured to death, and Vientiane sacked. By the time the French arrived, the only independent states in mainland Southeast Asia ("Indochina," a term most historians are using less and less because it implies that Southeast Asia is just a mix of India and China instead of its own region with its own specific historical processes) were Burma, Thailand, and Vietnam, and the Lao kingdoms were reduced to dependencies of Bangkok, either as tributaries or more directly integrated into Bangkok's empire. The only real exception was the Xiangkhoang Plateau, which was Vietnamese.
Without the French, Laos would be Thai.
So, well, without the French or any other foreigners taking direct control of the area (which would require a lot of PODs in itself) the dual division of the central and western mainland into the Thai and Vietnamese empires would continue. The majority of Cambodia and Laos would be just as Thai as Chiang Mai is today, at least for some time - European pressure would force Bangkok to eliminate vassal lords and install an unprecedentely centralized system, just as it did OTL.