For the meringue: Preheat the oven to 275 degrees F. Using a paper towel, grease a rectangular baking pan with the butter, making sure to get the bottom and sides. Sprinkle the pan evenly with the flour.
In a large mixing bowl, combine the vinegar, vanilla extract and egg whites. Using an electric mixer, beat until foamy. Slowly stream in the sugar and continue to whip the mixture until stiff peaks form.
Spoon the meringue onto the baking sheet, and delicately shape it into an even "crust" layer, bringing it up the sides of the baking sheet as well. Bake for 2 1/2 hours; turn off the oven and leave it in there for another 30 minutes. The dissipating heat will finish cooking the meringue. Remove from the oven and let cool.
For the lemon curd: In a large bowl, whip together 1 cup of the sugar, the lemon zest and juice and the egg yolks. Pour into a pot over medium heat and stir constantly until the mixture coats the back of a spoon, 5 to 7 minutes. Transfer to a bowl and cover with plastic wrap, pressing the wrap onto the surface of the curd; this will ensure it does not form a skin. Set aside to cool.
In another large bowl, whip the heavy cream with the remaining 1/4 cup sugar until stiff. Fold the cooled curd into the whipped cream. Layer the lemon mixture evenly over the cooled meringue. Cut into pieces and serve.