While treating for Cocci it is strongly advised to not offer vitamins until the treatment is over.Well, I wouldn't say I'm pumping her full of anything. And while Cocci protozoan do rely on some vitamins to thrive, so do the chickens - so it's also generally advised to continue vitamin support during treatment to assist in recovery.
At least in my experience and how I learned to rehabilitate birds.
The medication is usually effective enough disrupting protozoal DNA to counter any influence the presence of vitamins may have.