Preheat the oven to 250˚F and put the popcorn in a big bowl.
In a medium saucepan, combine the brown sugar, corn syrup, butter and salt and bring to a boil over medium heat, stirring until the mixture is well blended and uniform.
Reduce the heat and boil without stirring, swirling the pan occasionally, for 4 minutes.
Remove from heat and stir in the baking soda and vanilla—it will foam up at first.
Quickly pour over the popcorn; stir with a heatproof spatula or tongs to coat well, sprinkling with the bacon as you toss everything together.
Spread out onto a large rimmed baking sheet and bake for 30 minutes, stirring once or twice.
