Heat the oil in a heavy-bottomed pot over medium-high heat.
Once the oil is hot, add the kernels and close the pot with a tight-fitting lid.
Once the kernels start popping, carefully shake the pot so the kernels pop evenly and don't burn at the bottom.
If steam starts to build up, lift the lid slightly to release it.
As soon as the popping slows down, take it off the stove and let it sit for a minute, so any unpopped kernels pop.
Pour it into a large bowl and set aside.
Melt the butter in a saucepan over medium heat.
Once melted, stir in the sugar, corn syrup, and salt. Let it come to a boil, then allow it to simmer for 4 minutes—do not stir. Remove from the heat.
Remove from heat, then quickly whisk in the vanilla and baking soda.
The caramel will foam at this point. Quickly pour it over the popcorn, tossing until the caramel cools and starts to harden.
Spread the caramel-coated popcorn on a baking tray and bake at 250℉/120℃ for 40 minutes, carefully tossing it every 10 minutes to evenly distribute the caramel over the popcorn.
Remove it from the oven and allow it to cool.