Preheat oven to 375°. On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust to 1/2 in. beyond rim of plate; flute edge. Refrigerate while preparing filling.
Place pears in a large bowl; sprinkle with lemon juice and vanilla. Add cranberries. Combine brown sugar and flour; sprinkle over fruit and gently toss to coat. Spoon into crust.
In a small bowl, combine flour, oats, brown sugar and cinnamon. Cut in butter until crumbly. Sprinkle over filling.
Bake on a lower oven rack until filling is bubbly, 55-60 minutes, covering edge loosely with foil if needed to prevent overbrowning. Cool on a wire rack.
Dough for single-crust pie
Combine 1-1/4 cups all-purpose flour and 1/4 tsp. salt; cut in 1/2 cup cold butter until crumbly. Gradually add 3-5 Tbsp. ice water, tossing with a fork until dough holds together when pressed. Shape into a disk; wrap and refrigerate 1 hour.
Heat 3 tablespoons light corn syrup in microwave until warm; gently toss with 1 cup fresh or frozen cranberries, allowing excess syrup to drip off. Toss in 1/3 cup sugar to coat. Place on waxed paper; let stand until set, about 1 hour.