Cut 12-inch length extra wide heavy dtuy 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 nonstickcooking spray.
Toss peanut butter chips, baking soda and salt in medium heatproof bowl until baking soda is evenly distributed. Stir in sweetened condensed mik and vanilla. Set bowl over 4-quart saucepan containing 2 cups of simmering water. Stir with rubber spatula until chips are almost fully melted and few smaill pieces remain, 2 to 4 minutes. Remove bowl from heat and contine to stir until chips are fully melted and mixture is smooth, about 2 minutes.
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 pieces.