Im a 20 year old female 115 am i underweight? what vitamins can i take to gain weight?

Need more info: If you are short....5 feet 3 inches or so, 115 pounds may be a fine weight. This all depends on your bone structure and body fat. If you are taller, 115 may be underweight. Vitamins don't really help with healthy weight gain: best to eat frequent, every 2-3 hours, high caloric density food. This includes high protein and healthy carbs: whole wheat bread/cereal/pasta, and reasonable fat.