Preheat the oven to 325 degrees F (165 degrees C).
Mix the flour with 1/2 cup of sugar. Add the water gradually and cook on top of a double boiler for 15 minutes, stirring constantly, or until thickened.
Combine the remaining 1/2 cup of sugar with the yolks and salt and mix well with a whisk until the sugar is dissolved. Add the hot flour mixture to the yolk mixture gradually, mixing all the time. Return to the double boiler and cook for about 3 minutes more or until the mixture is thick and smooth.
Add the butter, extract, and vinegar. Mix well and remove from heat. Place a piece of plastic on top of this custard.
Meanwhile, beat the egg whites until foamy and gradually add the 6 tablespoons sugar. Beat until a stiff, glossy peak is achieved.
Pour the custard filling into the prebaked shell ( the custard should still be hot, if not, heat up a again before adding to shell). Top with the meringue. Spread the meringue all over the top of the pie, sealing to the edges of the crust. Place into the oven and bake until the meringue is a nice nut brown, about 15 minutes. Traditionally, this pie is served hot.