Commissar
Banned
as Ia Drang was in 1965, then the casualties shoot up to around 2500. As to the declaration of war point, it is irrelevant really. A simple declaration of national emergency and call up similar in scale and scope to Korea would be enough to ensure that the US has sufficient ground forces for Vietnam as well as its other commitments. I do agree however that there would have been a change in command, with McNamara and Westmoreland both potentially on the chopping block. The Chief of Staff of the Army, Harold Johnson, is on record stating he regretted not resigning after being turned down for the State of Emergency declaration. He would certainly do so in this case, and go to the press with his reasons... and LBJ would be hard pressed not to respond accordingly with either a complete withdraw or full scale mobilization and commitment. I tend to think that he would really have little political choice but to go all in (to use a poker term) and mobilize.
A national emergency call up is political suicide and Congress is already starting to realize it was duped over Tonkin Gulf. A disaster like this will have Congress launching all sorts of inquiries.
It will also be impossible to send more troops due to lack of logistics support at that time.
McNamara and Westmoreland are definitely finished as Ia Drang was their ideal and they wanted it as a way to fight a battle of annihilation like in WW2. A mindset they never dropped.