I see 5 openings:
(1) There are a lot of Lao in northeastern Thailand - they're called Isan - and one Lao prince asked for expansion in that region in 1945.
(2) Thailand annexed parts of Cambodia and Laos after an opportunistic war with France (1940-41) and had to give them back in 1946 to obtain UN membership. They may be able to retain some or all of what they had gained.
(3) The French did try to make Cochinchina a separate state, since it was a colony while Annam and Tonkin were protectorates.
(4) There are Khmers living in Cochinchina, so it's possible for Cambodia to annex some of it if it plays its cards right. This one works best combined with (3): first the French try to keep Cochinchina out of Vietnam, then they compromise with the Vietnamese to allow provincial or zonal referendums on whether the people want to join Vietnam or (presumably pro-French) Cambodia.
(5) There was talk of an Indochinese union in the late '40s, even among the communists.