Add oil to a heavy-bottomed pot and heat it 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, remove the pot from the stove and let it sit for a minute, allowing remaining unpopped kernels to pop.
Pour the popped corn into a large bowl and set aside.
Melt butter in a saucepan over medium heat.
Once the butter has melted, add the sugar, corn syrup, and salt and stir until just combined.
Once it starts boiling, let it simmer for about four minutes without stirring.
Remove from the heat, then quickly whisk in vanilla and baking soda.
At this point, the caramel will begin to foam. Quickly pour it over the popcorn and toss until evenly coated and the caramel cools and starts hardening.
Spread the caramel-coated popcorn on a baking tray and bake at 250℉ (120℃) for 40 minutes, tossing every 10 minutes. The caramel will melt again for the first 2-3 tosses (tossing coats the popcorn more evenly with caramel).
Remove the baking tray from the oven and allow the caramel popcorn to cool. Gently break into pieces and serve.