Pop popcorn and place in a very large bowl; set aside. Place brown sugar, Karo syrup, salt, and butter in a saucepan. Heat over medium heat until the butter has melted and the mixture is nice and bubbly.
Remove the caramel from the heat and add baking soda and vanilla, it will get all bubbly and foamy.
Pour the mixture over the popped corn and stir very well until all of the popcorn is covered.
At this point you can leave it as is for some nice, chewy caramel corn or you can bake it up to make it nice and crunchy. For crunchy caramel corn, bake on an oiled cookie sheet at 225 degrees for about an hour, stirring every 15 minutes, or until it reaches your desired level of crunch.