I've spent a lot of time in contemporary Vietnam, and I can assure you that Nightline went to the Delta with the full accompaniment of Communist Party translators, spies, and minders.
That is the way Hanoi works. Even today.
The Nightline press release on the topic - that first Hanoi said "no interviews" then "oh, OK, go right ahead and interview the locals" - is so transparently false that it's right up their with a Dan Rather statement. To wit, not plausible.
Yeah, it figures the last thing these totalitarians would do is allow foreigners unfettered access to anything or any one.