Does the dry skin look scaly, crusted, itch or stink? Is there hair loss around the dry areas? Is the fur greasy feeling? If the skin looks like this, you will probably have to take her to a vet. It could be a variety of things from allergies to mange. Probably your vet would simply look at it and maybe do a skin scrape, then if anything is wrong prescribe a medicated shampoo or maybe steroids such as prednisone. If it is just dry skin, there are special shampoos for dry skin and dandruff. If she's eating a good dog food, she shouldn't need a multi vitamin. If she's on a homemade diet, multi vitamins are a good idea just in case she's not getting good nutrition and that's causing problems. There are dog lotions and sunscreens, but I don't know much about them. If none of these things work, ask your vet.
I work at a vet clinic