Halve the watermelon. Scoop out the flesh, cut into bite-sized cubes, and return about 1/4 of the cubes to each of the 2 watermelon "bowls" you created.
Mix the vinegar, lemon juice, and yogurt in a small bowl. Whisk in the olive oil to emulsify. Add pepper and feta.
Top each bowl of melon with 1/4 of the dressing, then add another 1/4 of the watermelon to each bowl. Top with remaining dressing, and garnish with mint.