1. In a medium saucepan, saute onion and carrot on medium heat until softened, about 3 minutes.
2. Add broth, potato, thyme, garlic powder and salt. Bring to a gentle boil and simmer until the potatoes are soft, 15 to 20 minutes.
3. While potatoes are cooking, stir flour and milk together in a small dish.
4. Add chicken to the saucepan and stir 1 to 2 minutes until heated through.
5. Slowly add milk mixture and stir as the soup thickens. Boil gently for 2 to 3 minutes.
6. Before serving, add vinegar and pepper, if desired.
7. Refrigerate leftovers within 2 hours.