Melt the butter in a large pot over medium heat. Add the vegan marshmallows and stir until they have melted. Keep in mind, vegan marshmallows don't melt like traditional marshmallows. You will find they melt into a large blob. This is okay! Just keep stirring until each marshmallow is melted.
Remove the pot from the stove and add the vanilla. Make sure to do this while the marshmallows are still warm.
Add the brown rice cereal to the pot and fold into the melted marshmallows. Keep doing this until each piece of cereal is attached to the large ball of marshmallow.
Mix the sprinkles into the rice krispies. To do this, sprinkle as you fold the rice krispie mixture on top of itself. This is important and prevents the sprinkles from congealing in one spot. Don't go overboard as you can always add some more on top.
Line a 9 x 13 inch pan with parchment paper and press the vegan rice krispie treat mixture into the pan. I like to use my hands to do this as the mixture will stick to a rubber spatula. If desired, sprinkle some additional sprinkles over top!
Let it set in the fridge for about half an hour or out on the counter for an hour before slicing into squares.