Remove any un-popped kernels of corn from measured popped corn. Place the popcorn into a very large bowl.
Make a syrup from the butter, marshmallows and vanilla by heating them together and stirring in a large saucepan. Pour the marshmallow syrup over the popcorn in the bowl. Stir to combine and coat all of the popcorn with the marshmallow mixture. Then stir in the M&M's and peanuts until they are evenly distributed.
With greased hands (or waxed paper), firmly press the mixture into a well-greased tube or bundt pan. Allow to stand for 10 minutes; then loosen and turn out onto cake plate.