Pre-heat oven to 350 degrees
Place the chicken breasts on a baking sheet and brush them with olive oil and sprinkle with salt and pepper
Roast for 40 - 45 minutes until internal temp reaches 160 degrees
While the chicken is baking mix the yogurt, mayonnaise, tarragon, vinegar, salt and pepper in a small bowl and let rest so the flavors meld
Once chicken has cooled place remove the skin, take the meat off the bone, and cut into 1 inch pieces
Mix the grapes with the cut up chicken
Add half the dressing to the chicken/grape mixture. Mix well and add the remaining dressing in spoonfuls until you reach your desired creaminess. Save any remaining dressing.
Refrigerate the salad for at least 30 minutes and up to two days.
Taste the salad and add a bit more dressing, if needed. Adjust the seasonings to your liking and serve on a bed of lettuce or in a sandwich.