Cook sugars, butter or margarine, milk and salt to soft ball stage or until about 1/2 tsp of mixture dropped into a cup of cold water forms a soft ball. Add marshmallows and peanut butter just before removing from heat. Do not stir; cool to room temperature. Add vanilla and beat until mixture is creamy, thick and will hold its shape when dropped from a spoon. Pour into a well buttered 8 inch square pan. Cut into squares.