My knowledge on the star in Vietnam at least is each point represents a group of people: proletariat, peasantry, military, intellectuals, and petty bourgeoisie.

Not that many shapes are easily fashioned for producing crude flags. The Chinese flag represents the party with the big star and the four revolutionary (in the semi-feudal period) classes of the peasantry, proletariat, petty and national bourgeoisies.

Yellow has been a common secondary color for communist iconography and the star was an important regimental symbol. The two overlapped over time.

I've heard it symbolises the communist party