Start with the blackberry syrup. Mix together the ingredients for the syrup, except for the vanilla, into a saucepan.
Heat on medium-high heat and mash the blackberries. Cook until the liquid thickens to a syrup.
Strain out the blackberry bits, and allow the syrup to cool.
Stir in the vanilla.
Start on the lemonade. Squeeze the lemons, making sure to remove any seeds.
Mix the water and lemon juice together.
In your pitcher, or in your glass, add blackberry syrup to reach your preferred sweetness.
Notes
Only use freshly squeezed lemon juice. I recommend investing in a nice juicer. I recommend leaving the whole lemons in hot water for 10 minutes, then gently rolling on your countertop before you juice them.