To the extent that any tactic remotely associated with the French at Dien bien phu could be said to be not entirely stupid or insane, the French employment of the the Chaffees would be a good candidate. Fire support is what tanks do. If you're creating a fortress with the intention of drawing your enemy into the open where you can destroy him with fire then having armoured, tracked machines which can carry cannon and machine guns and thus be relatively less vulnerable to suppressing fire and using them in local counter-attacks would be helpful. Having a full regiment of tanks would have been more helpful, assuming that we can magic away the logistics of bringing them in (you may recall the French needed to disassemble the Chaffees just to fly them in) and supplying and maintaining them.
A study of the ground, both via maps and contemporary photographs, will show that the Viet Minh positions were primarily in the surrounding hillsides, which were forested. The VM artillery was rather quite well concealed and protected. They studied techniques employed to great effect by the Japanese in the second world war.
An attack, uphill, in fairly well vegetated ground, by just 10 tanks, would have been unlikely to achieve much. Armour needs to co-operate with other arms, especially infantry. The French would have needed to attach at least one or two battalions of their already small reserve, which would have probably gotten pinned down by the VM artillery, surrounded, then destroyed by the VM, which had about twice as much infantry as the French. It would be quite probable that the Viet Minh would have used 'Death Volunteer' sappers in their close-in attacks on the tanks themselves.
The French air force was hard pressed enough to carry out its attempts to bombard Viet Minh positions and to re-supply the garrison. It would not have had any extra capacity to co-operate with such a proposed operation. I don't think that the 1954 U.S.A.F. had much capability that would be similar to that of the Second Indochina war's Arc-light so assuming an alternate universe in which the Yanks help openly like that is rapidly a topic for the SF stack exchange site.
The French did use armour in the First Indochina war, often as part of battle groups called Groupement Mobile. In the Second Indochina war, the Free world forces (Americans, ARVN, and Australians among others) and the North Vietnamese used tanks. There was a famous small tank battle in 1969 and the 1972 and 1975 offensives saw the NVA use armour extensively.
Martin Windrow's The last valley might be an informative read. Simon Dunstan's Vietnam tracks has a chapter on armour in the First Indochina war. It has a fair number of photographs which are interesting.
The competence of the Viet Minh staff work would have been sufficient to plan for predicted fire missions on the French outposts and forming up positions and for hitting any force as it made its way to a hill. There would have been some field telephones in the VM trenches and they could use flares and runners if need be. The co-ordination would not be very good - perhaps Great war Hill 70 level – but would you want to be there?
If the attack takes place during the day the observation of the forming up of the French attacking force could be made from the hills as well as from the VM trenches. Spotting of shot would be from there as well. Tanks are noisy and the assembly of them would be a signal that they were going into action and the direction of attack. This holds true for day and night.
If the attack would take place at night there could be no co-ordination with the French air force as they didn't fly at night. The spotting of the VM artillery would be more difficult but stuff like star shell and WP are useful for this sort of situation.
You will recall that it is several hundred metres between the French outposts and the hill sides and it is mostly cultivated ground, without tree cover. During the siege the VM were busy building saps and a rather significant force of VM infantry could be assembled to close action. I would not put it past the VM to infiltrate men behind the main body of the attacking force and their start line. Assuming that any significant French force got to a hill side, it would then need to climb the hill under fire and then try to storm and destroy the dug-in artillery positions. As mentioned, the VM learnt from the Japanese about how to dig in artillery so just finding the dang things, let alone destroying them would be difficult. Then the French would need to try to do that for all of the rest of them.
And you do understand that Chaffees were light tanks for recce don't you?
What actual study of the matter have you made and what are your findings?