CGTN has some great ones on China. Here's a couple:
Gay Leap Forward: LGBT in China
Not a documentary, but this is a great interview on life in the GDR from an American who lived there: https://youtu.be/Oy8CrizjKh4
Luna Oi is of course the best English language source on Vietnam:
How do ELECTIONS work in VIETNAM?
For Cuba, Azure Scapegoat has some good stuff:
Cuba: Before and After the Revolution - The Story of When Michael Parenti Visited Cuba (this one is EXCELLENT)
¡Las Sandinistas! is a pretty decent documentary of the Sandinista revolution in Nicaragua from women who were there.
Also, I haven't yet seen it, but Thomas Sankara: The Upright Man is supposed to be good.