Mix popcorn and peanuts in a large bowl or roasting pan. Keep warm in a 250°F oven.
Melt butter in a 2 quart saucepan. Stir in sugar, corn syrup and salt. Bring to a rolling boil on medium heat, stirring constantly. Boil without stirring for 5 minutes. Remove from heat and stir in soda and vanilla.
Quickly pour over warm popcorn while tossing until well coated. Spread in a shallow baking pan or roasting pan.
Bake at 250°F for 45 minutes. Stir every 15 minutes. Make sure every piece of popcorn has been moved around on the bottom.
Remove from oven and turn out onto foil to let cool.