> many Vietnamese went to the US [..] they can see firsthand the benefits of living in the US
This applies to many countries. Why does it specifically influence Vietnamese perceptions more?
> Vietnamese are fiercely capitalistic
This is true, but again, they're hardly alone. Chinese are fiercely capitalistic as well - and in the same context as the Vietnamese, within the framework of a nominally communist society. Why doesn't your theory apply there, too?
I think the situation is much more nuanced than these too-obvious theories, or that can be surfaced in superficial "attitude" surveys. As mentioned in other comments, Vietnamese are deeply (and IMO rightfully) suspicious of China, despite the state's theoretical status as an ally. The USA, despite its history of botched and ham-handed interventions, is ultimately seen as a friend. There's a love/hate aspect to that attitude too, similar to Filipino attitudes to some extent. The absence of military bases in-country helps by keeping the notion theoretical, rather than the often-negative daily experience of having foreign soldiers around a la Korea and Japan.
Anyway, without getting too much into the weeds of theorising and speculation, my point was that it's complex - more complex than the OP suggested. Nostalgia for the South Vietnamese flag is a niche sentiment at best, and attitudes to the USA are a complicated product of history and pragmatism. "90% positive" does not capture the nuance, in my opinion.
It's probably too late but it would have been good to hear from some non-emigrant Vietnamese.