In a large glass bowl, combine lemons, sugar and salt. Cover and let stand overnight at room temperature., Preheat oven to 450°. On a lightly floured surface, roll one half of dough to a 1/8-in.-thick circle; transfer to a 9-in. cast-iron or other ovenproof skillet. Trim even with rim. In a large bowl, whisk eggs until frothy; lightly brush over crust. Add lemon mixture, flour and vanilla to eggs; mix well. Pour into crust. Roll remaining dough to a 1/8-in.-thick circle; place over filling. Trim, seal and flute edge. Cut slits in top., Bake 15 minutes. Reduce oven setting to 400°. Bake until crust is golden brown and filling is bubbly, 20-25 minutes longer. Cover edge loosely with foil during the last 10 if needed to prevent overbrowning. Remove foil. Cool on a wire rack; serve warm.