Grease an 8x8-inch baking pan and line it with parchment paper. In a large pot over low heat, melt the vegan butter. Add the marshmallows, vanilla, and salt, stirring until smooth, about 3 to 4 minutes.

Remove from heat and fold in the cereal until evenly coated.

Scrape the mixture into the prepared pan and press lightly into an even layer with a buttered spatula. Let cool at room temperature for 30 minutes or refrigerate for 15 to 20 minutes until firm.

Lift the slab out by the parchment, cut into 16 squares, and serve.