Diet has a very limited affect on egg shells, it is much more genetic. And unless you are starving them, it really has none. What you are feeding them is good enough. I have never given the vitamins in my life.
People who are a bit inexperienced worry about odd eggs, and I think that is because they have always boughten store bought eggs which are highly sorted. Eggs with meat spots, or rough shelled, or soft shells are not used. Instead these eggs are sold to production companies that use them in some product they are producing like cakes.
One of the problems on BYC is that people do not post about normal every day stuff. They post about concerns, so that often times, new posters start to worry about all these things that can go wrong. And the wrong tends to be the exception not the rule.
So don't race out there, let it go. They may eat a few, but it won't cause egg eating.
Mrs K