The question of routinely giving dogs vitamins comes up fairly regularly....  while any medical advice needs to come from your vet and you follow that, here is some general information. (Since dogs and humans are both mammals, this information is relevant to both).
        Vitamins are catalysts, chemical "helpers" that aid in the body's metabolism. They are naturally occurring in nature, in the foods that are eaten. They are either fat or water soluble. They are required in certain amounts (and these amounts are very small compared to normal measurements we use everyday).  No matter how much of the water soluble ones you take in, you cannot get too much since the body will just eliminate them through the kidneys and urination. This is why "megadoses" of vitamins generally are not useful as they just get eliminated - along with your purchase price of them. Fat soluble ones on the other had get absorbed into the body's tissues and large excess amounts can be stored up this way. Even though these are "vitamins" too much can indeed become toxic just like anything else. Illness can arise from this excess toxicity that can be very difficult to diagnose, since many people never think to tell the doctor that large doses of vitamins are being given or taken since they are supposed to be "good for you" and not considered to be a medicine (or worse). Vitamin toxicity can be very serious and indeed fatal. The same thoughts apply for herbs. You can and people (and animals) DO get into problems with not letting the doctor know that "natural herbs" are being used as again they don't think of them as medicine. Herbs can interact with other medications to cause enhanced or harmful effects.... or even do so on their own in certain circumstances. For example, Gingko Biloba has become a very popular and common herb that is taken - yet, it also acts as a blood thinner so people on prescribed blood thinners taking gingko can find themselves in big trouble with major bleeding problems..... even those not on anticoagulant medication need to not be taking gingko for a period prior to surgery to avoid excessive bleeding at time of surgery. This is an clear example of why any doctor needs to know any and all supplements that are being taken or given regardless of whether you think that they are medicine or not.
        In general, the old basic thought of eating a balanced, well rounded diet is still the best course. Vitamin or other supplements should not be needed if following a good nutritional plan and consuming a healthy well balanced diet....    you don't need the extras and as has been explained can well be just wasted purchases. If there are problems with eating or getting a good well rounded balanced diet then consult a doctor or certified nutritionist and follow their instructions and prescribed amounts of needed supplements, which of course comes down to be sure to ask and follow the advice of your doctor or vet.