In a large skillet cook potatoes in hot oil over medium heat about 15 minutes, or until tender and brown on all sides, turning occasionally. Spoon potatoes into serving bowl. Sprinkle with the salt and pepper.
To same skillet add summer squash. Cook and stir over medium heat for 3 to 5 minutes or until just tender. Add to the potatoes in the serving bowl.
Pour vinaigrette dressing over potatoes and summer squash. Add sweet pepper, tomatoes, parsley, and thyme. Toss gently to mix. Cool.
Cover and chill for 4 to 24 hours, stirring salad occasionally. Top with thyme sprigs, if desired. Makes 6 side-dish servings.