Line a 9x13-inch pan with wax or parchment paper and grease well.
Heat a large skillet or saucepan over low heat. Add the marshmallows, cake mix, and vanilla.
Stir frequently as the marshmallows melt until the mixture is smooth. Quickly fold in the rice krispies to the marshmallow mixture until combined and everything is coated.
Transfer sticky cereal mixture to the prepared pan, pressing down with the back of a greased spoon or your fingers until the top is level.
Allow the krispie treats to set, at least 15 minutes before cutting and serving.