CGTN has some great ones on China. Here's a couple:

China's war on poverty

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:

Is Vietnam a Police State?

Is Vietnam Socialist?

How do ELECTIONS work in VIETNAM?

For Cuba, Azure Scapegoat has some good stuff:

How Democracy Works in Cuba

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.