Preheat oven to 400°. With an electric mixer, beat together Neufchâtel cheese and granulated sugar until fluffy. Add flour, egg and vanilla extract, then mix at medium speed until blended. Set aside.
Roll out pastry sheet on a lightly floured surface to a 16-by-11-inch rectangle. Transfer to a large baking sheet. With a short end toward you, spoon cheese mixture lengthwise down center third of dough. Spoon jam in a line down center. With a knife or pastry wheel, cut 5 horizontal slashes on each side of filling, beginning on outside of dough, cutting toward middle to within 1/2 inch of filling. Crisscross strips over filling, overlapping for a braided effect.
Bake pastry until puffy and golden, about 30 minutes. Let cool for 15 minutes on pan on rack. Dust with confectioners' sugar just before cutting crosswise and serving.