For the lilac flower syrup, pluck the flowers from the umbels. Be very careful when doing this. Dissolve the sugar and citric acid in the water and boil the solution for about 10 minutes.
Then stir in the vanilla sugar. Let the syrup cool down a bit. When it is only lukewarm, add the lilac flowers. Leave this mixture to infuse in a cool place for 4 to 5 days.
Stir once or twice a day. At the end of the infusion time, strain the syrup through a strainer. Do not squeeze, otherwise the juice will become cloudy.
Boil the syrup again briefly. Then pour into boiled bottles, close immediately and leave to cool upside down.