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