Pulse pecans in a food processor until finely chopped. Remove pecans.
Add flour and sugar to processor; pulse twice. Add butter; pulse until crumbly. Combine water and vanilla; pour through food chute with processor running. Process just until dough begins to form a ball.
Add chopped pecans; pulse twice or until pecans are incorporated.
Sprinkle hands with flour, and shape dough into a smooth ball; cover in plastic wrap, and chill at least 20 minutes.
Roll dough to 1/4-inch thickness between sheets of plastic wrap; fit into a 9-inch pieplate. Trim excess pastry from edges; crimp edges, if desired. Prick bottom and sides of pastry. Cover and chill 30 minutes.
Line pastry with aluminum foil or wax paper, and fill with pie weights or dried beans.
Bake at 350° for 20 minutes. Remove weights and foil; bake 2 more minutes or until lightly browned. Cool on a wire rack.