As I recall, the French did a poor job of understanding the terrain in the area. The basic concept was to 'draw the VM out' of the mountains and force them to fight in the open ground around the French base. They essentially chose to build a defensive position in the least defensible place available.
The site they chose for their base was surrounded by higher ground, which allowed the VM to place observed artillery fire onto most areas of the base. They underestimated/
dismissed the VM artillery, deciding that they could not move heavy artillery into the area because of poor/no roads. The VM simply manhandled the guns into position (in some cases disassembling the guns and moving the parts, then reassembling the gun in position) , then supplied them by hauling ammo, etc. by foot.
The French failed to ID and control the major avenues-of-approach, and ceded the initiative by building a defensive position and essentially daring the VM to take them out. Once in position, the French did not have the forces either inside or outside the valley to protect the logistical routes. The VM quickly cut these routes and isolated the forces at DBP. From there on, the siege is a study in creative logistics how things can go wrong. Virtually every problem with logistics, primarily air-supplied, can trace it's root to the poor planning of the operation and the poor choice of location. In the end, the logistical inability to support the beseiged base led to the defeat. The French ran out of virtually everything before surrendering. Even the Foreign Legion needs to eat, and their guns need ammuntion just like everyone elses. No bullets, no fight.
Much of this poor planning has to fall on the shoulders of the French staff, which was largely composed of WWII vets and traditionally trained officers. The French were disdainful of the VM, and failed to understand the enemy capabilities. The focus was on drawing the VM into a set-piece battle, where the natural superiority of the French warriors could crush the savages. The French staff assumed that the VM would react as they determined, and never considered that the VM might find alternate courses-of-action that were not accounted for in the planning. Failing to incorporate the needs for logistical support and contingencies into the base plan allowed an unsupportable course-of-action to be chosen. and led to the deaths of a lot of good men.
Dunno if this helps, but there you go...
