The diaspora in Western countries are mostly made up of people who left VN as political refugees. In the USA, those who fly the SVN are either:
People and their descendants who left VN in 1975 with the help of the US administration.
People who were admitted to the USA from the refugee camps in HK, Singapore, Malaysia, Indonesia, Thailand and the Philippines. They left Vietnam clandestinely mostly by boat and were boarded by merchant ships crossing them on their way to commercial harbours.
People and their descendants who underwent imprisonment in Vietnam because they were workers in the South Vietnamese administration, they served in a US admin or organisation, they got granted access to the USA under the Ordered Departure Program.
You understand why those are flying the SVN flag and not the current Vietnamese flag.
The immigrants who settled in the US recently with workers visa, marriage or green card against investments …are loyal to the communist flag.