In a small mixing bowl whisk together oil, vinegar, salt, and pepper. Remove 1 tablespoon of the mixture and brush it over the tenderloin. Reserve remaining oil and vinegar mixture for use in Melon-Tomato Tumble.
Meanwhile, for Melon-Tomato Tumble, cut melon in half. Cut one of the melon halves into bite-size pieces. Cut remaining melon half into 4 wedges; cover and chill wedges in the refrigerator until serving time. In a medium bowl combine the melon pieces, tomatoes, onion, and mint. Add remaining oil and vinegar mixture; toss to coat.
Remove the pork to a cutting board. Cover and let pork rest for 10 minutes. Slice meat on the diagonal and serve with the tomato mixture spooned over melon wedges. Makes 4 servings.