Coat a 6-quart slow cooker insert with nonstick spray.
Toss pears, cranberries, 1/2 cup sugar, cornstarch, lemon juice, vanilla, and cinnamon in cooker.
Whisk flour, baking powder, salt, and remaining 1/4 cup sugar. Cut in butter until pea-size pieces remain.
Stir in milk just until shaggy dough forms. Drop 10 rough spoonfuls over fruit, leaving gaps.
Stretch a clean cotton kitchen towel over cooker; cover with lid. Fold towel ends away from heat.
Cook on high for 2 1/2 to 3 hours until fruit bubbles and a biscuit center reaches 200°F.
Uncover; rest 15 minutes to thicken juices.