Line an 8x8 or 9x9 pan with parchment paper and lightly grease it. In a large pot over low heat, melt vegan butter.
Add vegan marshmallows and stir continuously until fully melted and smooth. Remove from heat and stir in vanilla extract.
Quickly add rice cereal and mix until fully coated.
Press mixture into prepared pan evenly without packing it too tightly.
Let set at room temperature before slicing into squares.