Combine the blueberries and vanilla sugar in a large heavy saucepan. Add 1 cup water and the lemon juice and bring to a boil over high heat, stirring occasionally. Reduce the heat and simmer for 15 minutes, or until the mixture begins to thicken.
Remove from the heat and strain through a fine sieve. Cool to room temperature, cover and refrigerate.
2006, Ina Garten, All Rights Reserved