For the filling:
Cook 4 cups blueberries, sugar, cinnamon, water, and lemon zest in Dutch oven over medium-high heat, stirring occasionally, until mixture is thick and jam-like, 10 to 12 minutes. Whisk lemon juice and cornstarch in small bowl, then stir into blueberry mixture. Add remaining blueberries and cook until heated through, about 1 minute; remove pot from heat, cover, and keep warm.
For the topping:
Combine buttermilk, butter, and vanilla in measuring cup. Whisk flour, baking powder, baking soda, salt, and 6 tablespoons sugar in large bowl. Slowly stir buttermilk mixture into flour mixture until dough forms.
Using small ice cream scoop or 2 large spoons, spoon golf ball-sized dumplings on top of warm berry mixture (you should have 14 dumplings). Wrap lid of Dutch oven with clean kitchen towel (keeping towel away from heat source) and cover pot. Simmer gently until dumplings have doubled in size and toothpick inserted into center comes out clean, 16 to 22 minutes.
Combine cinnamon and remaining sugar in small bowl. Remove lid and sprinkle dumplings with cinnamon sugar. Serve immediately.