Shouldn't hurt, when we take vitamins, the excess is just excreted out when we go to the bath room. Besides most vitamins only have micrograms of the vitamins in them.. You may want to dissolve them first in some hot water. If you dump a bottle of them in your compose pile, they may just form a big clumb and take forever to dissolve do to all the various powders used to make
them tablets. Let us know how yur experiment turns out.....