Cut 12-inch length extra-wide heavy-duty aluminum foil; fold edges back to form 7 1/2-inch width. With folded sides facing down, fit foil securely into bottom and up sides of 8-inch-square baking pan, allowing excess to overhang pan sides. Spray foil with nonstick cooking spray.
Toss chocolates, baking soda, and salt in medium heatproof bowl until baking soda is evenly distributed. Stir in sweetened condensed milk and vanilla. Set bowl over 4-quart saucepan containing 2 cups simmering water. Stir with rubber spatula until chocolate is almost fully melted and few small pieces remain, 2 to 4 minutes.
Remove bowl from heat and continue to stir until chocolate is fully melted and mixture is smooth, about 2 minutes. Stir in pecan. Transfer fudge to prepared pan and spread in even layer with spatula. Refrigerate until set, about 2 hours. Remove fudge from pan using foil and cut into squares.