LINE 13 x 9-inch baking pan with foil; grease lightly.
MICROWAVE morsels and sweetened condensed milk in large, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Stir in vanilla extract. Fold in marshmallows and nuts.
PRESS mixture into prepared baking pan. Refrigerate until ready to serve. Lift from pan; remove foil. Cut into pieces.