Preheat oven to 350. Prepare brownie mix as directed on package; stir in nuts. Spread into greased 13 x 9 pan. Bake 25 to 30 minutes. In heavy saucepan, over medium heat, melt chips and margarine with sweetened condensed milk and vanilla. Spread evenly over brownie layer. Cool. Chill 2 hours or until topping is firm. Cut into bars. Store covered at room temperature.