I've lost a considerable amount of weight. Is there any type of vitamin that will help me regain my appetite?

Weight Loss. Your considerable weight loss is abnormal and you should consult your physician who can find out what is the cause of considerable weight loss and then advice you. Vitamins will not be any help. So consult your medical doctor and do not fool around.
With or with diet? Lf you are loosing weight with out diet or any weight control program, please consult a physician, too many conditions, very important have to be check when that happens. If you was doing a diet, B12 is good vitamin. I suggest to check wth your dr. Any others vitamins deficiency.