In a medium size saucepan, combine the blueberries and the water, bring to a boil, reduce the heat and simmer for 15-20 minutes. Pour the blueberries through a sieve set over a bowl, pressing down on the blueberries, mashing and stirring to extract the juices. Discard the solids and measure the liquid. It should be about 3 cups of juice, if not, add enough water to equal 3 cups.

Return the blueberry juice to the saucepan, add the sugar, salt, lemon strips, lemon zest, lemon oil and bring to a boil, stirring to dissolve the sugar. Boil the mixture over medium-high heat until it registers 225°F on a candy thermometer, about 20-25 minutes.

Remove from the heat, discard the lemon zest strips and add the butter and the vanilla, stirring to incorporate well. Allow the syrup to cool, and pour into a one-pint mason jar.

This syrup makes about 2 cups and will keep, refrigerated, for up to 6 months.