In a large bowl, mix the milk and the yogurt or starter sachet, and then whisk until you obtain a smooth mixture. Cut the vanilla pod in two and scrape out the seeds with the tip of a knife. Add them to the mixture and mix well.
Pour the mixture into the bottles. Place them in the yogurt maker and cook in yogurt mode for 6 hours.
Once the programme is complete, close the bottles if you have not yet done so and place them in the refrigerator for at least 4 hours. Shake well before drinking.