Preheat the oven to 350 °F. Place the sugar and butter in a heavy-based (oven-proof) round pan over medium–high heat. Stir two or three times and then simmer gently until the sugar is melted and golden. Add the water to keep the sugar from burning. Add the pears, lemon juice, and vanilla to the pan and gently stir until the pears are well coated. Cook gently for 5 minutes. Remove the pan from the heat and arrange the pears so that they are lying as flat and close together as possible.
Cut out a circle of pastry that is 1" wider than the width of the pan. Lay it over the pears and tuck in the edges so that everything is well covered. (If you did not use an oven-proof pan, transfer the cooked pears to a round shallow 8-9" baking pan) Make 4 or 5 small slits in the pastry with a knife to allow steam to escape during cooking. bake for 30-40 minutes or until the pastry is puffy and golden.
Allow the tart to cool for a few minutes before turning out onto a plate by holding the plate over the pan with pot holders and invert the pan. Be very careful, as the caramel is extremely hot. Serve warm with vanilla ice cream on top!