Don't vitamins dissolve in water? I wouldn't count on it providing grit. In any case, most established worm bins have plenty of grit to go around.

I'm not sure what goes in multivitamin tablets. There's plenty of inert material in there besides the active ingredients. It probably won't do the worms any harm in the short term. I just don't know what happens in the long term as material builds up in the bin. It probably won't matter if you're talking about a dozen tablets once a year.

Andrew