https://www.pewresearch.org/global/2015/06/23/1-americas-global-image/
America’s image is mostly positive among the Asian nations polled. Particularly large majorities see the U.S. favorably in the Philippines (92%), South Korea (84%) and Vietnam (77%).
It's from 2015 but more recent polls have found similar data IIRC
https://www.brookings.edu/articles/comparing-global-views-of-the-united-states-and-china-during-the-trump-and-biden-administrations/
79% of people polled in Vietnam chose the US in the "Preferred alignment choice in U.S.-China rivalry" which is a different metric obviously but still points towards the same idea
Edit: According to Luna Oi, Pew Research Center is a neoliberal think tank that cannot be trusted on anything about Vietnam
https://youtu.be/hPCoDz_CPCc
I'm from Vietnam but I migrated to America in early 2010s for work. I can only tell you what I known from back then. There was no openly pro south Vietnam people in Vietnam. If those kinda people even dare to show their true color i.e. display the flag or openly call for the revival for South Vietnam, they would've been shut down really fast and have an express line to jail for reeducation purposes.
That doesn't mean that those people don't talk anonymously online or behind closed doors though. The economic problems in Vietnam after the war also didn't help with building confidence in people who was directly benefited from the southern government. In fact many older South Vietnamese still have a rosy colored picture of the puppet regime. But at least those people are dying out and with them any passion for that fascist regime.