To make cake: Preheat oven to 350°. Grease and flour three 8-inch cake pans. Sift the flour, baking powder and salt into one bowl. In another bowl, whisk together the vegetable oil, yogurt, sugar, egg yolks, lemon zest and vanilla. Slowly blend the dry ingredients into the wet ingredients. Whisk the egg whites until stiff but not dry. Fold into yogurt mixture. Pour into prepared pans and bake 30 to 35 minutes. Cool for 10 minutes and invert onto cake racks. Cool completely.
To make lemon curd filling: Sprinkle the gelatin over 1 tablespoon of the lemon juice in a small bowl. Cook the remaining lemon juice, sugar and salt in a saucepan until the sugar dissolves and the mixture is hot but not boiling. Whisk egg yolks in a bowl. Whisking vigorously, slowly add the hot lemon-sugar mixture to the egg yolks. Return the mixture to the saucepan and cook over medium-low heat, stirring constantly, until mixture reaches 170° on a candy thermometer. Remove the pan from heat and stir in the gelatin mixture until dissolved. Stir in the butter. Pour through a fine-mesh strainer into a clean bowl. Press plastic wrap directly onto surface and refrigerate until firm, at least 4 hours (this lemon curd filling can be prepared the day before).
To make meringue garnish: Place a Silpat (available in baking supply stores) on a heavy baking sheet. Preheat oven to 200°. Beat the egg whites until frothy, then add the cream of tartar and continue beating. Gradually add the sugar while beating, until whites are very stiff and glossy. Sift powdered sugar and fold in to meringue. Spread onto prepared baking sheet. Bake for 2 to 2 1/2 hours or until dry. Cool. Break meringue into small pieces (this meringue garnish can be prepared the day before).
To make icing: Place powdered sugar, limoncello, butter, vegetable shortening, salt and vanilla in a mixing bowl. Whisk at high speed for 10 minutes, scraping down sides, until light and fluffy.
To assemble cake: Place 1 cake layer on a serving plate. Brush 1/3 of the limoncello onto the cake, then spread 1/2 of the lemon curd filling onto the cake layer. Repeat with the second layer. Place the third layer on the cake, then brush the remaining limoncello on the cake layer. Spread the top and sides of the cake with the icing. Place the meringue garnish on the top and the sides of the cake.