Grease bowl of 4- to 6-quart slow cooker with butter; set aside. Toss together blueberries, plums, brown sugar, cornstarch, orange zest, orange juice, cinnamon and nutmeg. Scrape into slow cooker.
In a large bowl, whisk together flour, sugar, baking powder, cinnamon and salt; cut in butter using pastry blender or fingertips. In a separate bowl, whisk together egg, milk and vanilla until blended; stir into flour mixture just until dough starts to come together.
Drop spoonfuls of dough over filling. Cover and cook on High setting for 3 to 4 hours or until filling is bubbly and biscuits are cooked through. Let stand for 15 minutes.
Serve with vanilla ice cream.
If using frozen blueberries, rinse before using.