If you need any special vitamins, your doctor will prescribe them. Make sure that vitamins do not contain agents, such as artificial colors, to which you are sensitive. Some people search for a diet treatment for asthma as an alternative to taking medicines. But no such diet exists and sadly, much time, money and suffering are wasted in the search. If you have allergies that trigger your asthma, you will naturally avoid those foods.
Some individuals are sensitive to sulfites, which are used as preservatives in many foods.
But there is no other dietary control for asthma. Many people resist taking medicines, especially when they are feeling well. Some look at it as a sign of weakness. Some fear side effects, or feel it's too expensive. You need to discuss your feelings about your treatment with your doctor.