Preheat the oven to 250 degrees. Put the popcorn on a large baking sheet or roasting pan.
In a medium saucepan, combine the oil/margarine, brown sugar and corn syrup. Cook over medium high heat until mixture boils. Cook for another 2-3 minutes. Remove from the heat and stir in the baking soda and vanilla (mixture will bubble and foam). Pour the hot mixture over the popcorn and stir well making sure to cover it all in caramel.
Put in the preheated oven and bake for 45 minutes, stirring occasionally. Remove from the oven and cool. Store in an airtight container.