Cut the plums in half and remove the pits. Sprinkle half the sugar on the bottom of a frying or sauce pan just big enough for all the plum halves to fit in a single layer, cut side facing down. Sprinkle them with the remaining sugar, drizzle with lemon juice, and then cover with lid and let sit for 12 hours or overnight.

Uncover and add 1 cup of water and the star anise to the pan, and bring the mixture to a boil over medium high heat. Reduce heat to a simmer and cook for 30 minutes. Remove from heat and add the vanilla extract and cool to room temperature. Place clean mason jars, seal with a lid, and place in the refrigerator. Will keep for several months.