Preheat oven to 250º
Prepare a large cookie sheet with parchment or spray with oil.
Spread popcorn evenly on sheet
Combine sugar, butter and syrup in large sauce pan.
(You'll be sorry if it's too small)
Bring to a boil, stirring frequently
Boil 5 min
Cool 1-2 min
Whisk in soda and vanilla. The caramel will expand to fill your pan.
With rubber spatula, pour caramel, coating popcorn evenly
Bake 1 hour, turning several times to keep coating popcorn evenly.
Remove from oven and separate popcorn as it cools to make it easier to eat/share.