HEAT oven to 250 degrees F.
COMBINE sugar and syrup in small saucepan. Cook over medium heat, stirring constantly, until mixture begins to boil. Remove from heat. Blend in peanut butter and vanilla.
PLACE popped popcorn and peanuts in a large bowl. Pour peanut butter mixture over popcorn, stirring until coated.
PLACE popcorn in a single layer on a foil-lined cookie sheet. Bake 10 minutes, stirring after 5 minutes. Remove from oven and cool.