In a medium size saucepan, combine the blueberries and the water. Bring just to a boil and then 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 zest, lemon juice, lemon oil and bring to a boil, stirring constantly 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 and discard the lemon zest strips. Add the butter and the vanilla and stir to incorporate well. Allow the syrup to cool and pour it into a one-pint mason jar.
This syrup makes about 2 cups and will keep, refrigerated, for up to 6 months.