For the Croutons:
Preheat the oven to 400 degrees F. In a medium bowl, toss together the bread cubes and melted butter. Add the vanilla sugar and toss until the bread cubes are coated in sugar. Arrange the bread cubes in a single layer on a parchment-lined baking sheet and bake until golden brown, about 8 to 10 minutes. Cool completely.
For the Syrup:
In a small saucepan, bring the orange juice, lemon juice, and sugar to a simmer, over medium-low heat. Stir until the sugar has dissolved, about 2 to 3 minutes. Add the blueberries and strawberries and simmer until the fruit softens, about 6 to 8 minutes. Cool the syrup to room temperature.
For the Ricotta:
Combine all the ingredients in a medium bowl. Mix well.
To serve: Divide the ricotta mixture between 4 decorative dessert bowls. Spoon the fruit syrup over the ricotta and top with the croutons. Garnish with the sliced mint.