Bring berries, 1/2 cup sugar, 1/4 cup water, the vanilla seeds and pod, and a pinch of salt to a simmer in a saucepan over medium heat. Cook, covered, for 5 minutes. Whisk together cornstarch and remaining tablespoon water; add to berries. Simmer for 1 minute. Let stand while making dough.
Mix together remaining 2 tablespoons sugar, the flour, baking soda, grated nutmeg, and 1/8 teaspoon salt. Rub in butter using your fingers until very small pieces remain. Stir in buttermilk until dough forms.
Return berry mixture to a gentle simmer over low heat, and add 6 mounds of dough. Cook dumplings, covered, until cooked through, about 10 minutes. Remove pod. Serve warm.
Serving idea: Top slump with ice cream.