OTL they briefly occupied it, but only as a show of power and Chiang did not want to hold it, despite the Americans telling them to keep it.
That said, if China wants to hold it, the French are simply going to swallow up this one blow.
If this happens- I would assume initially China's clients are Laos and KMT backed VNQDD-led "North Vietnam" with its capital in Hanoi ruling north of the 16th parallel. British forces occupying south of the 16th parallel would turn over "South Vietnam", Cambodia and far southern Laos to the French. The Chinese would back the VNQDD declaration of independence.
The US China Lobby would support backing Vietnamese independence at this time. Not as a matter of highest priority, but it would be the group, and the Republican Party's, stated preference.
Europeanists and Atlanticists in the State Department would support continued French ties with Indochina in order to not alienate France. So would British interlocutors with Americans.
VNQDD would support insurgents in the French zones.
The French would have to cut deals with other local Vietnamese and colonial factions being in a weaker diplomatic and military position with China trying to exert more influence via supporting an independent Vietnamese client state. So the French, unlike OTL, will not try shenanigans like trying to formally separate Annam from Cochinchina. They will have to make considerable autonomy promises to Vietnamese willing to talk to them and try to continue to sell continued French Union membership involving a customs union and trade preferences, a military protectorate, and French oversight of Vietnam's foreign policy, but much more opportunity for politically ambitious Vietnamese in local administration than the pre-war regime.
The French will be forced to promise all this even if delivery comes up short, and will also have to use China and Sinophobia as a foil, basically making the argument that any government set up and left behind by the Chinese can't be truly independent and any pro-Chinese or Chinese-backed party like the VNQDD can't be true patriots, because they have Chinese imperialist stink and cooties on them.
The only other way it might go down would be if the Chinese occupy it all and then French don't get a chance to set up any footholds.
If France is seen unable to reestablish itself in either northern Vietnam, or in any of Indochina, as of 1945-46, because of Chinese obstruction, what will be the net domino effect on the larger French Empire?
Does it stimulate the Algerian revolt and war and French concession of independence there to happen earlier?
Does it change nothing and that struggle starts in earnest only in 1954 like OTL?
Or does the lack of France being seeing fighting and then losing at Dien Bien Phu in '54, slow down the emergence of the FLN insurgency, and slow down the concession of Moroccan and Tunisian independence in 1954, and leave France with more endurance to persist in putting down Algerian revolt for an extra decade or so into the 70s or 1980?