Yes, at the end of the day it's all about who has the power. I just wanted to point out that present Vietnam is far from united and that you can't imagine how much resentment there is in Saigon towards the communist government in the north.
Saigon had a "good" war, barely saw any fighting, but the post-war was horrible, scarring southerners to this day, many people repeatedly tried to flee on dingies out of desperation from human (northern) caused starvation and scarcity, this in a land that had never experienced hunger (the Mekong Delta is extremely fertile and milked by the north). They haven't forgotten.
The world is full of hopeful irredentists, and whether they will ever succeed or not is not a matter of right, but might (maybe in the shape of a foreign power). It is just that in southern Vietnam most people are irredentists, which I suspect is not the case in the US south, isn't that right?
From what little I know, both the center of the country and the south, party and non-party alike, generally feel that the northern elite is clannish and doesn't allocate jobs and investment fairly, which is kind of a big deal for a country with such a large state sector.
In the American case most of the state of Mississippi did not celebrate the 4th of July until 1945 because they considered themselves to be occupied confederate territory for 81 years: https://www.visitvicksburg.com/blog/celebrating-independence...
I can well understand some feeling persisting a mere 65 years later.
That said, for American southern irredentists of past, present and future, they're stuck with the confederacy alone, which is inextricably connected to slavery even for people like Robert E. Lee who was personally but not politically opposed to slavery. That's simply a much harder thing to identify with morally and practically, especially when 30-40% of the population of the south are descended from freed slaves.
The Republic of Vietnam flag stands for the hundreds of thousands of ARVN soldiers and administration and their families, but it also stands for things like a catholic dictatorship attacking temples in a 90% buddhist country, rigged elections, coups, and being the junior partner of a foreign power that carried out a brutal and dehumanizing counter-insurgency.
Unlike the American south, the southern Vietnamese have a much older identity and much richer history to draw upon. So, I wonder if in time a prior symbol/flag/something from their distinct history will become more resonant for them in the future, which conveys the identity without the baggage.