LINE 13 x 9-inch baking pan with foil; grease lightly.
MICROWAVE morsels and sweetened condensed milk in large, microwave-safe bowl on HIGH (100 percent) power for 1 minute; stir. Microwave at additional 10- to 20-second intervals, stirring until smooth. Stir in vanilla extract. Fold marshmallows and nuts.
PRESS mixture into prepared baking pan. Refrigerate until ready to serve. Lift from pan; remove foil. Cut into pieces.