Line a 9"x13" pan with parchment paper.
Place the cereal in a large mixing bowl.
In a saucepan over medium heat, add the vegan butter, corn syrup and chocolate chips. Heat until melted and creamy, stirring often and making sure not to burn.
Remove the pan from heat and stir in the vanilla.
Pour the mixture over the cereal. Stir until evenly coated.
Mix in the marshmallows, 1 cup at a time, until evenly mixed.
Pour the mixture into your prepared baking dish. Press down so that it's evenly spread across the pan.
Let stand until firm (at least 1 hour).
Cut into 15 large bars or up to 48 small bars. Enjoy!