Stir baking soda, water and vanilla together in a small bowl. Set aside.
Butter two cookie sheets and a large metal or other heat proof bowl. Place these items in the oven at 200 degrees to keep them warm.
Combine sugar, water and corn syrup in a 3 quart sauce pan. Place a candy thermometer in the pan. Cook over medium heat until the temperature reaches 240 degrees on the thermometer.
Remove the pans and bowl from the oven. Pour chex and peanuts into the bowl.
Stir the butter into the candy when it reaches 240 degrees and continue to cook, stirring constantly until the temperature reaches 300 degrees.
Remove candy from heat and stir in the baking soda mixture. At this point the candy will foam.
Pour candy over the cereal and peanuts, stirring to try to coat everything with the candy. Work quickly and pour it all onto the warm cookie sheets. Press it out as best you can onto the pans.
Allow to cool completely.
Place in plastic zip bags and break by hitting with a butter knife.