France cannot win the First Indochinese War by arms alone, in fact their best course of action is to avoid the war entirely. Which is emminently possible, numerous books discuss their options ad nauseum.
But if you must go down the path of the French fighting the first Indochinese war... then the best option to present a sizable advantage to the French would be have their 1946 operation to decapitate the Viet Minh be successful. Have the paratroopers kill Ho Chi Minh and General Nguyen and you will give the French a sizable advantage.
However, the upper cadres of the Viet Minh were excellent as well - so the advantage will last for a year or two until a successor is established. Which may be enough time for the French to organise Vietnamese independence. Regardless of the path chosen the Vietnamese ultimately gain independence.