I sincerely doubt that the Vietnamese government has any idea what would or wouldn't be useful for ABC or John Kerry. I suspect their part was a simple "give them full cooperation." ABC supplied the script.
No. The "independent producer" who speaks fluent VN and has had total access in and out of VN for the last 25 years is the key. He understands USA politics, and made the overtures to the VN govt about how to coach the "witnesses" for maximum Kerry advantage. He is well enough established in VN that the VN govt bought his plan and went with it.