Place water and granulated sugar in a medium saucepan, stir to combine, and bring to a low simmer over medium heat.
Simmer, stirring occasionally, until the sugar has completely dissolved, about 5 minutes. Remove from the heat and set the simple syrup aside to cool to room temperature.
Place orange juice and evaporated milk in two separate bowls. Divide the simple syrup and vanilla extract between the two bowls (1/2 cup simple syrup in each bowl and 1 teaspoon vanilla extract in each bowl), and whisk to combine.
Pour the mixtures into two separate pans, cover with plastic, and place in the freezer.
After 1 hour in the freezer, scrape the partially frozen granita mixtures with a fork, breaking up any large chunks. Repeat this step 2-3 times, every hour, until the granitas are frozen solid.
Spoon the granitas into bowls or glasses, creating a layered look. Garnish (optional) with orange slices and mint sprigs before serving.