Preheat the oven to 250 F.  Pop your popcorn into a giant bowl.  An air popper is definitely preferable for this, as you don't want to have oil of any kind on the popcorn.

Melt the butter in medium saucepan, then add the brown sugar, corn syrup, and salt.

Bring to a boil, stirring constantly.

Boil for 4-5 minutes, not stirring.

Remove from the heat and stir in the baking soda and vanilla.

Pour the caramel over your popcorn in the big bowl and stir together to try and somewhat coat the popcorn (it won't be 100% covered and that's fine).

Spread over a couple of large rimmed baking sheets and bake for 30-40 minutes.  Make sure you stir it every 10-15 minutes to make sure it doesn't stick, and to continue spreading the caramel around.  We use metal baking sheets and a thin metal spatula to do that.

Let cool, then break apart into bite-size pieces and clumps.  Store in an airtight container at room temperature (moisture is the enemy).