You could have the French give independence to the protectorates of Annam and Tonkin but keep the colony of Cochinchina. Would that be enough? If not, then somehow get Cochinchina to become independent but ruled by the white minority, kinda like South Africa and Rhodesia. No idea why the local French would push for this, maybe in reaction to the métropole trying to introduce majority rule, ceding the colony to Vietnam or going communist.