As long as you are tying the correct preventive visit ICD-9 code to the vision screening CPT, I would think no modifier should be necessary.
You might want to tell your Medicaid carrier that per CPT 2011, pg. 33, "Vaccine/toxoid products, immunization administrations, ancillary studies involving laboratory, radiology, other procedures, or screenings tests (eg, vision, hearing, developmental) identified with a specific CPT code are reported separately."
Best o'Luck!