Let's see now.....
Since it's creator, the 4th French Republic wasn't all that stable, I'd say that the best way for the French Union to survive would be if these things happened.....
Have De Gaulle become President of the 4th French Republic. Rallying around a war hero would help strengthen France's shaky government.
Have him engage in certain government reforms that helped to stabilize it on a long term basis.
Have the Nationalists in China win the Chinese Civil War or at the very least, limit the Communists to Northern China in the aftermath. This might limit or eliminate support to the rebel groups in French Indochina and allow France to hold onto it and save face.
Increase the amount of financial aid to France in the aftermath of World War II to help ensure that it recovers faster.
Have the French make certain concessions to the natives throughout France's colonies that lead them to want the French to stay such as autonomy and development of their lands.
Have the French government play the whole "we're a bulwark against Communism" dogma in order to get additional U.S support which might be hard since the Communists were a significant factor in French politics at the time.
Even with all of this, I'm unsure if the French Union could have survived to the Present Day even without Vietnam as France was badly weakened by all of the fighting in World War II.