I'm on a low calorie diet to lose weight but when I exercise (walk), I feel tired sometimes. Do I need to add a vitamin supplement?

Coconut oil. Coconut oil is high in medium chain fats that are easily burnt for energy. I tell my patients to add a tablespoon to at least one meal a day. Beware of low fat or fat free diets. Good fats are required for optimal health.
See below. You may be experiencing a drop in blood sugar. An engergy drink will do better than a vitamin supplement.