No command approval required, sought, or desired.
The Rules were clearcut....simple...direct...and the decision was left up to the Aircraft Commander.....remembering ours was a Transport Helicopter and not a Gunship.
I cannot speak to what rules they had.....beyond some areas that were remote basically uninhabited...and known NVA base areas.....were designated "Free Fire Zones" and anything moving was fair game.
You must remember the context the Rules were designed under....once main force North Vietnamese forces came south and began to operate....the War morphed from a typical insurgency and began to take on more conventional war kinds of confrontations between regular forces of each side.
There were still large unpopulated areas of jungle in those days and it made for something similar to fighting as in Burma, New Guinea, the Solomons.
There are a lot of misconceptions about the nature of the fighting when some try to compare Vietnam to Malaya and Borneo.
Small things such as Tanks, Artillery, Radar Controlled AAA, SAM's, MIG 15/17/21 Fighters that the Opposition forces in Malaya did not have.