So, with a PoD anywhere between 1900 and 1950, how could Vietnam unite with China, perhaps with a degree of autonomy in the package?
How strong were sinophile elements within Vietnam in this time frame, IOTL?
I hear there were plans to transfer control of Indochina from France to KMT China, but those were discarded after the Viet Minh leadership protested. Ho's "better smell French than Chinese shit" quote comes to mind, but there are rumours that the quote was fabricated as pro-French propaganda...
If a union between China and Vietnam does happen, i'd expect it to be under the aegis of the Kuomintang as ruler of China. But could a CCP-ruled China still accept the prospect of uniting with Vietnam? Despite their "anti-imperialist" mindset, they still annexed Tibet rather than installing a puppet communist regime in Lhasa, after all.
(So, the more plausible preconditions could be a stronger KMT in China and less exclusively communist participation in the Vietnamese struggle against Japanese occupation.)
What would be the economic, political, and cultural effects of a Chinese Vietnam?
How would the Vietnamese regional economy perform in the context of integration with China? What of land reform, commerce, etc?
Assuming China can have a military presence in Vietnam, how does this affect its power projection capabilities in the South China Sea, and its relationships with neighboring countries?
Any cultural ramifications of an annexed Vietnam on the Chinese? Would Vietnam become a source of internal migration for the republic?
Vietnam would rather shot itself than being annexed by any foreign power. No if, no but - anyone surrendering will be scorned forever. That was what happened to a Vietnamese king in mid-17th century (I think), he kneeled to the Chinese emperor to ease down the invasion threat. Result? It takes a few hundred years for some people to consider that a "fair price" (note that there was no serious/major invasion after all).

If the CCP and/or the KMT wanted it, they could steam roll over Vietnam, especially if the French decided to just GTFO. However, none of the VCP would accept that, they would simply replace "French" by "Chinese" in their shitlist and keep fighting.
I think it's telling that as WW2 was winding down and the Japanese were clearly going to lose, discussions about the fate of Indochina were becoming more commonplace among the Allies. At one point, FDR asked Chiang Kai Shek if it were possible for China to take over Vietnam in the event France prove too busy with other commitments, or France being flat-out unable to keep Indochina safe. China was chosen as the closest power in the region and a former occupier of Vietnam.

Chiang's reply was telling: "Under no circumstances!"

Needless to say, the whole matter was dropped quickly because the Chinese clearly didn't want that headache.