Because if you spend enough time on the internet, especially the Western side, you start to hear voices of people who oppose their governments and not enough from those who support it.
Even many who are supportive tend to say that their government is authoritarian.
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I don't know for real but I think maybe most of party members are in the northern part. The south used to be heavily colonized by the west before it got liberated by northern folks. The situation here might be similar to Shanghai in China.
To elaborate on this, after the French Indochina war in 1954 due to the treaty with France, most of the party members in the South had to move into the North, many of them didn't get to come back South until 1975. The Southern fascist regime also hunted down and killed a lot of our Southern comrades during the war. The lead to a big lost in the Southern branch of the party capability of organization and leadership.
In urban area where the control of the puppet regime is the strongest, they also had a lot of propaganda to brainwash the mass into anti communism. After the war, many of those same suburbanite also saw the biggest lost of quality of life due to the drop in free money from US aid and new sanctions which reinforced those past propaganda. That's why you see a tendency for Southern suburbanite to be skeptical to hostile toward leftist ideology and have a rosier picture for capitalism.
I'm also a Southern Vietnamese here to answer your questions. Party members are still required to study Marxism-Leninism. You are actually tested in how well you know ML ideologies before your application to the party is approved. But deal to history, most of the core members of the politburo is from the North and the Northerner are typically more hard line ML while many Southern party members flirt with socdems and liberal ideology.
When he mentioned liberalism I think he mean the ideology and not the culture war in the West. Most people living in those socialist countries still have to struggle mostly with consequences of colonialism and capitalist imperialism and is less embroiled in identity politics like in the West. Vietnam and Cuba is still better than most of the US regarding LGBTQ issues and gender equality though.
o7