Place 9 cups Chex cereal in a large mixing bowl and set aside.
In a microwave-safe bowl, combine 1 cup chocolate chips, ½ cup creamy peanut butter, 1 tablespoon vegetable oil, and 1½ teaspoons vanilla extract.
Microwave on high for 30 seconds. Stir well, then continue microwaving in 15-second intervals, stirring after each interval, until the mixture is completely melted and smooth.
Pour the melted chocolate-peanut butter mixture over the cereal. Gently stir until the cereal is evenly coated, being careful not to crush the cereal.
Divide 2 cups powdered sugar evenly between two 1-gallon zip-top bags.
Divide the coated cereal evenly between the two bags. Seal each bag securely and shake gently until the cereal is completely coated with powdered sugar.
Spread the puppy chow onto a large baking sheet and allow it to cool completely.
Once cooled, transfer the puppy chow to a large bowl. Add 4 cups Golden Grahams, 2 cups mini marshmallows, and 3 Hershey's chocolate bars cut into pieces.
Gently stir until all the ingredients are evenly distributed.
Store in an airtight container at room temperature for up to 1 week.