Pound sugar in a mortar with vanilla seeds.
Pears cut into 4 parts. Remove the core and cut into slices. Sprinkle with lemon juice. Transfer to a 3 liter jar.
Bring 1.2 liters of water and sugar to a boil in a saucepan, add the remaining lemon juice. Pour boiling syrup over pears. Cover with lids.
After 5 minutes, drain the syrup, bring it to a boil and pour the pear syrup over again. Repeat this step one more time.
Seal the jar, turn it upside down and leave to cool completely.