In large bowl, mix flour, 1/2 cup sugar, the salt, lemon peel and vanilla bean seeds. Beat in butter with electric mixer on medium speed until mixture looks like small peas. In small bowl, beat egg yolks and 1/4 cup limoncello with whisk. Slowly beat egg mixture into butter mixture. Turn off mixer as soon as dough comes together. Roll into a ball, and wrap in plastic wrap. Refrigerate 1 hour.