Grate the zest of 2 oranges; set aside. Juice the oranges and whisk with 1 1/2 cups Greek yogurt, one 14-ounce can sweetened condensed milk, 2 tablespoons lemon juice, 1 teaspoon vanilla and a pinch of salt. Remove 1/4 cup of the mixture to a small bowl and stir in the orange zest. Pour the plain filling into a 9-inch graham cracker crust. Spoon the zest mixture over the top, then swirl with the back of the spoon. Cover and freeze until set, about 4 hours.