Heat oven to 350°F. Grease 13x9-inch pan. In large bowl, combine cake mix, oil and eggs; blend well. Stir in chocolate chips, vanilla chips and candy bar pieces. (Mixture will be thick.) Press half of mixture in bottom of greased pan. Bake at 350°F. for 10 minutes.
Meanwhile, in medium saucepan, combine margarine, caramels and condensed milk. Cook over medium-low heat until caramels are melted and mixture is smooth, stirring occasionally.
Remove partially baked crust from oven. Slowly pour caramel mixture evenly over crust. Crumble remaining cake mix mixture over caramel.
Return to oven; bake an additional 25 to 30 minutes or until top is set and edges are deep golden brown. Cool 20 minutes. Run knife around sides of pan to loosen bars. Cool 40 minutes. Refrigerate 1 hour. Cut into bars. Store in refrigerator.