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 black pepper.
In the same skillet add summer squash. Cook and stir over medium heat for 3 or 5 minutes or just until tender.
Add to the potatoes in the serving bowl. Pour vinaigrette dressing over potatoes and summer squash. Add sweet pepper, tomatoes, parsley, and snipped thyme. Toss gently to mix and cool.
Cover and chill for 4 to 24 hours, stirring salad occasionally. If desired, top with thyme sprigs.