Place chocolate in large microwavable bowl. Add sweetened condensed milk; mix well. Microwave on HIGH 2 to 3 minutes or until chocolate is almost melted, stirring after every minute. Stir until chocolate is completely melted. Stir in vanilla and 3/4 cup of the pecans. Pour into foil-lined 9-inch square baking pan.
Microwave caramels and 1 teaspoon water in small microwavable bowl on HIGH 1 minute; stir until caramels are melted and smooth. Drizzle over top. Swirl with small knife for marble effect. Sprinkle with remaining 1/4 cup chopped pecans, pressing gently into fudge.
Refrigerate 2 hours or until fudge is set. Use foil to lift out of pan onto cutting board. Cut into 36 pieces. Store in tightly covered container at cool room temperature.