Spray a 9x13 pan with non-stick butter spray.
In a large pan melt your butter on low heat. Once melted, stir in the bag of mini marshmallows until they are mostly melted. Then stir in the vanilla.
Combine the Lucky Charms and rainbow sprinkles into the marshmallow mixture. When fully combined transfer your mixture into the 9x13 pan.
Spray a spatula with the non-stick butter spray to flatten out the lucky charms mixture in the pan.
Let the treats set for about one hour before cutting into squares.