Combine blueberry juice and jam sugar in a large pot.
Rinse blueberries and place in pot.
Rinse jars with hot water and drain on a clean tea towel.
Add grated lemon zest and lemon juice to taste, along with vanilla bean, and bring to a boil. Simmer until berries are soft but not falling apart and jam has reached gelling point. Remove vanilla bean.
Using a ladle or measuring cup, fill jars with hot jam up to the rims. Wipe rims clean and immediately seal tightly. Invert jars until vacuum-sealed, about 5 minutes. Then turn jars right-side up.