Make the mint vinaigrette by combining the white balsamic vinegar, honey and salt in a bowl, stirring to dissolve the salt. Add the olive oil and whisk until emulsified. Then stir in the mint. Chill the vinaigrette until you are ready to serve.
To Make Ahead – Combine the diced watermelon and blueberries. Make the mint vinaigrette and chill. At least 30 minutes or up to 4 hours before serving, add the feta cheese and toss with the vinaigrette. This is best eaten the day it is made and doesn’t store well as leftovers.