Preheat the oven to 400, line baking sheet with parchment paper and lightly grease it.
Combine flour, sugar, and salt in a bowl of an electric mixer. Chop cold butter into small pieces and add it to the flour.
Start mixing on low until it looks like coarse crumbs. Add cold water and keep mixing until dough comes together. Transfer onto a work surface and knead a few times. Cover in saran wrap and pop in the refrigerator for about 30 minutes.
In a medium mixing bowl, combine blueberries, sugar, lemon juice, lemon zest, and scraped vanilla beans. Mix well and set aside.
Separate the cold dough into 4 even pieces. On a very lightly floured surface, roll out each dough into a circle, about ⅛ of an inch thick (you may also want to lightly flour the rolling pin). Carefully pick up one dough circle and place it on the greased parchment paper.
(Mix the filling again.) Spread a quarter of blueberry filling in the center of the dough circle, leaving a little less than an inch space uncovered around the edges.
Fold edges of the dough up over the blueberries, all around the circle, folding the dough onto itself on the side. Press the folds gently, so they don't come apart.
Repeat with the remaining three dough circles and blueberry filling. Place mini galettes about two inches apart from each other.
Sprinkle some vanilla sugar all over the folded edges of galettes. (You can also use brown sugar or white granulated sugar.)
Bake for 20-25 minutes, until the crust is golden.