Preheat the oven to 60°C (145°F/Gas mark ¼) and line a tray with baking paper. For the popcorn, heat the oil in a large saucepan. As soon as the oil is hot, add the popcorn kernels. Swirl the pot once to coat the kernels in oil, then seal with a lid. As soon as the first kernels start to pop, reduce the temperature and give the pot a shake every now and then.
Once you hear no more kernels popping, take the pot off the heat and spread the popcorn on the baking tray. Remove unpopped kernels. Keep the popcorn warm in the oven.
For the caramel, combine baking soda and salt. Heat the sugar and vanilla sugar, caramel syrup, butter and 2 tablespoons water in a large pot until the sugar has dissolved. Do not stir. Once the sugar has dissolved, swirl the pot once and leave the sugar to caramelise over medium heat for about 5 minutes. Remove the pot from the heat and stir in the baking soda mixture. Add the popcorn to the pot as soon as the mixture starts to foam a little. Stir well until the popcorn is well coated with the caramel. Return the caramel popcorn to the tray and quickly separate the individual popcorn pieces. Leave to cool fully and store in an airtight container.