They are not the vocal minority. Based on my personal experience, at least half of college students, who I talked to, support the Vietnam's government whole-heartedly. A quarter will always criticize for whatever the government do. The rest don't care. Decades and decades of propaganda work .. pretty well. They have been brainwashing since they started going to school at 6 y/o. Personally, I only began to recognize the lies of VN government in my early 20s. The lack of focus on critical thinking in national curriculum contributes a significant part to this. In addition, while highly corrupted and lacks of transparency, the government of Vietnam also has done many things right, e.g., rejoining the world after cold war, economic development, covid19, cheap public education, and public healthcare to some extends.
The alternative views are no good. Except a handful of good eggs, most of democratic activists occasionally spread fake news, half-truth stories, and conspiracy theories. The majority of them are Trump's supporters and anti-vaxxers. I'm not saying that all Trump's supporters are bad, just stating the fact. They are often discredited by the government's supporters due to this and the current situation in the US.
Last but not least, the government of Vietnam is pretty confident in suppressing free press because China can get away with it without any real pressure from the US and EU. At the end of the day, if there is an alternative to Facebook, the situation will like be the same due to the current political landscape of Vietnam. Twenty years ago, it was Yahoo's blogs. Even if FB withdraws from Vietnam tomorrow, there will be another platform and things will be the same.
Sorry for English mistakes.