Add the Rice Chex and pretzels to a large bowl. Drizzle the melted butter and vanilla over the top, then toss until everything is lightly coated.
In a small bowl, mix together the sugar and cinnamon. Sprinkle it over the cereal mixture and toss again so every piece gets that sweet, cinnamon flavor.
Spread the mix out on a baking sheet in an even layer. Bake for 20 to 25 minutes, stirring halfway through so it toasts evenly.
Let it cool completely for maximum crunch, then fold in the cereal marshmallows and serve.