With a pastry cutter or potato ricer begin to incorporate the sugar and butter. Combine flour until resembles course crumbs.
Add water, vinegar and vanilla (if you choose) and combine until it holds together when pressed.
Press into a 9-inch flan or spring form pan. Cover the bottom and sides evenly. Chill for 15 minutes.
Pierce pastry with a fork, cover with foil and bake at 375 for 20 minutes remove the foil and bake until lightly golden.
In a small saucepan combine remaining 1/4 cup sugar, lemon zest and orange juice. Simmer over low until zest is softened and translucent and sugar has entirely dissolved. Once done, remove from heat allow to cool and then add to fresh lemon juice.
With an electric mixer mix 3 egg whites and 1 egg yolk with 1/2 cup sugar. Blend until creamy. Add remaining 2 yolks. Blend.
While mixing with a spatula, drizzle in warm lemon juice into egg mixture.
Fill pastry shell and bake at 350°F for 20-30 minutes until firm but not too golden. If the top is browning, cover with foil.