Searching for Vietnam time, I see sites stating that Vietnam follows Indochina
Time, so this view does not appear too controversial in the region.
I agree from the design point of view that countries should have their own
independent time zones, as what they do is so unpredictable, and should not be
allowed to affect any other country, so allowing a little judicious duplication
would be in order, where there is no political commonality but only geographical
proximity or coincidence in decision making.

I noticed also that timeanddate.com represents Hanoi incorrectly for the period 1970-1975.
See attached screenshot.

Timeanddate.com is usually a good site for time zone history after 1970, and often goes beyond of what tzdb offers.
But here they neither included backzone nor the comment which advises to follow Asia/Bangkok for half of Vietnam.

That problem would easily be fixed if Asia/Hanoi were moved from backzone to file asia. All sites following the main distribution of tzdb would update. Nobody can really be expected to study the comments, to get exect timestamps for after 1970.