PREHEAT oven to 350° F.
COMBINE graham cracker crumbs, butter and sugar in medium bowl; press onto bottom of ungreased 13 x 9-inch baking pan.
MICROWAVE 1 cup morsels, sweetened condensed milk and vanilla extract in medium, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. The morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until smooth. Pour evenly over crust. Top with nuts and remaining morsels.
BAKE for 20 to 25 minutes or until edges are bubbly. Cool completely in pan on wire rack. Cut into bars.