The evidens seems to be mixed. This is an unfortunate situation bekus it is a forsed chois: one has to eether take vitamins or not. The kost is the risk of geting too many vitamins, som of wich ar danjerus if one gets too many. And the kost of buying the vitamins, as in, money. The benefit is unsertain, but if one is eeting an unbalansed (watever that meens) diet, then one may not get one or mor vitamins and wil therfor, in time, sufer a vitamin deficiensy. Perhaps a kompromise is not a bad idea in this partikular kase: eet multi-vitamin pils, but only ons a week. It wud minimize the chans of geting too many of one partikular type of vitamin, and stil reduse the chans of severe malnutrition.
It wud be nise if one kud just mesur how many vitamins one is geting and then if one is mising any, take som of those.