Preheat the oven to 250 degrees. Butter the bottom and sides of a large roasting pan. (Depending on the size, you may need two pans).
Pop the popcorn in two batches: Heat 2 1/2 Tablespoons of canola oil and four corn kernels in a large kettle with a lid over medium high heat. Once the test kernels pop, remove the pan from the heat, and add 3/4 cup corn kernels. Put the lid on, and keep the pan off the heat for 30 seconds. Then place back on the heat. Once the kernels really start popping, shake the pan intermittently.
When there are 2-3 seconds between pops, remove the pan from heat and pour the contents into the roaster. Repeat to pop the other half of the corn.
Make the caramel: In a heavy bottomed saucepan with a candy thermometer clipped to the side, heat the buttery spread, brown sugar, and corn syrup over medium heat. Stir constantly, until the candy thermometer reads 255 degrees F.
Remove the pan from the heat, and add the vanilla extract, baking soda, and salt. Stir to combine.
Quickly pour the caramel over the popcorn, and stir to coat. Be careful! The caramel is super hot.
Place the caramel corn in the oven, and bake for about one hour, stirring every 15 minutes.
Let caramel corn cool completely - it will crisp up as it cools off.
Store in an airtight container.