For the crust: Pulse the flour, sugar and salt together in a food processor. Add the butter and pulse until the mixture resembles a coarse meal with some pea-size lumps. Drizzle in the vinegar and 4 tablespoons of the water; pulse until a dough starts to come together. If the mixture is too dry, add more water, a teaspoon at a time.
Turn the dough out onto a piece of plastic wrap, form into a disk and wrap well. Refrigerate until firm, at least 1 hour.
For the filling: Beat the cream cheese, egg yolk, 2 tablespoons of the granulated sugar, and the vanilla in a bowl with an electric mixer until smooth.
In another bowl, toss the cherries with the cornstarch and the remaining 1 tablespoon granulated sugar.
Finish the galette: Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Lightly dust a work surface with flour and roll the dough out to a 12-inch round. Transfer to the prepared baking sheet.
Spread the cream cheese mixture evenly over the dough, leaving a 1 1/2-inch border. Spoon the cherries over the cream cheese, leaving any excess juice in the bowl. Fold the edges of the dough over so they just barely cover some of the filling.
Beat the egg white with a splash of water and brush it over the crust. Sprinkle the Demerara sugar evenly over the filling and crust.
Bake until the crust is golden brown, 30 to 35 minutes. Cool for 10 minutes before serving.