If *post*-unification time - which is post-1970 - doesn't match Asia/Bangkok, then Asia/Bangkok should not be used for any part of Vietnam; there should be one or more Asia/{whatever} for Vietnam from 1970 to the present.
To be a bit more precise, for our purposes, I mean when the clocks were reunified, independent from the various stages of political reunification. Our data has that occurring on 1975-06-13. Since that time, all of Vietnam has agreed with Asia/Bangkok, so there is no issue there.
If Vietnam's pre-unification offset or rules were different between the north and the south from 1970 to 1976, then "one or more" means "at least two", e.g. Asia/Hanoi and Asia/Ho_Chi_Minh (even if it was called Saigon in 1970).
This, however, is where the need for a second zone for Vietnam arises. And indeed there are already two listed in zone1970.tab (Asia/Ho_Chi_Minh and Asia/Bangkok); it's just that one of them happens to be modeled by a city outside Vietnam. That is not considered a problem per theory.html.