Purchase some fresh chick starter. Then mix it into existing feeding so 1/3 of consumed is chick starter. The starter will be more heavily fortified with vitamins relative to fomulations for older chickens at time of manufacture. Mix should get you back into good vitamin content.
If you feed birds on vitamin deficient diet (I said diet, not feed as not always =), then look for a loss of egg production first. Then you will see a reduced interest in feeding followed by a loss of weight and thriftiness (feathers and flesh will look pale or shabby). Death can follow although I suspect you will be allowed to pony up funds to by feed before that.
This is one of those situations where fermenting part of the feed or providing fresh dark greens can help a lot. Fermentation mediating fungi and bacteria can make some of the B vitamins I would be be most concerned about by storage at elevated temperatures and too long. Do not ferment all of the feed fed to them as the process can degrade other vitamins. The greens will also have what is needed but care must be taken not to dilute protein and energy intake too much. Most diet formulations for egg production are designed to be cost effective under optimal conditions. That means they not overly fortified with respect to very expensive protein as a means to control feed cost.
