In a medium saucepan over low heat, melt the unsalted butter. Once melted, add the entire package of Campfire Marshmallows and stir continuously until they are completely melted and smooth. Remove the saucepan from heat and stir in the pure vanilla extract.
In a large mixing bowl, combine the Very Berry Cheerios and the dehydrated mixed berries. Pour the melted marshmallow mixture over the cereal and berries. Using a spatula, gently fold everything together until the cereal and berries are well coated.
Transfer the mixture into a greased 9x13-inch baking dish or one lined with parchment paper. Press the mixture firmly into the dish, ensuring it is evenly spread out. If desired, sprinkle the Campfire Mini Marshmallows on top and gently press them into the mixture. Allow the treats to cool completely before cutting them into squares.
