Pre-heat your oven to 250°F.
Place the popcorn in a large, deep bowl (divide it into two large bowls if you can't quite fit it all in one). Stir the pecans in with the popcorn if using.
In a medium sized saucepan, melt the butter over medium heat.
Stir in the the brown sugar, corn syrup and salt. Stir constantly until the mixture comes to a boil.
When the mixture starts boiling, stop stirring and allow it to boil for 4 minutes.
Stir in the baking soda and vanilla.
Pour the caramel over the popcorn and quickly stir it to coat it as evenly as you can.
Transfer the popcorn to a large baking sheet and spread it out in a thin layer.
Bake for an hour, stirring every 12-15 minutes.
Remove the pan from oven and allow the popcorn to cool completely before breaking it up and serving.