Pour the entire gallon of milk into the Instant Pot. Add the yogurt and vanilla. Whisk to combine. Put the lid on the pot.
Press the Yogurt button on the Instant Pot. The timer will automatically be set for 8 hours.
The pot does not actually come to pressure on the Yogurt setting so feel free to remove the lid and taste test the yogurt around the 8 hour mark. If it is not quite tart enough, you can continue to cook the yogurt for another hour.
When the yogurt is done, remove the inner pot, cover with plastic wrap, and place the pot in the refrigerator. Do not stir the yogurt or transfer it to another container. Allow the yogurt to set up in the refrigerator for 8 hours.
After the 8 hour refrigeration period you may transfer the yogurt to another container. I store mine in 1/2 gallon size mason jars but any type of container will work.
At this point, a sweetener of your choice can be added in. My favorite sweeteners are maple syrup and raw honey. Other great mix ins include granola, cinnamon. nuts, fruit syrup, or fresh fruit.