Line an 8x8 inch baking dish with parchment paper.
In a large saucepan or skillet, melt the butter and add vegan marshmallows over low heat until the marshmallows are just melted. Stir in the vanilla to combine.
Remove the pan from the heat, add the Rice Krispies and mix until evenly coated with the marshmallow mixture.
Transfer the mixture into the prepared pan. Sprinkle the top with chocolate chips if you like. Coat a spoon or spatula in non-stick cooking spray and use it to press the mixture firmly into a packed, even layer.
Allow the pan to set at room temperature for at least 30 minutes. Remove the treats from the pan and slice into squares.
Store the treats in an airtight container at room temperature for up to 5 days.
Notes
To make peanut butter rice krispie treats, add 1/4 cup creamy peanut butter to the pan with the vanilla extract.