merchant center keeps disapproving our vitamin products

If the products themselves look fine, check the site too. Missing contact details, no returns or refund policy, or shipping info that's hard to find can get you flagged for misrepresentation, and that one hits the whole account rather than single products. Also check the reviews and testimonials on the product pages. A customer saying it cured something reads as a health claim on your page, even though you didn't write it.

what reason does it give in diagnostics? that decides everything. with supplements it's usually one of two things. either an ingredient is on google's unapproved substances list, in which case you can't advertise that product, or it's the claims. words like cure, treat, prevent, detox, or anything that sounds like a medical promise in the title, description or on the landing page will trip it. google checks the page too, not just the feed. if it's claims, tone the copy down on both, then request a review on the disapproved items. don't just resubmit unchanged, repeated rejections on the same thing can escalate to an account level problem.