If an optometrist finds a vision impairment do they have to give you your record?

Record ownership. Your record is in the custody of the practitioner who examines you. They are legally obligated to give you a copy if you request it or to forward a copy to another practitioner if you request that also. From an optometrist you would want your eyeglass prescription and the contact lens prescription mostly as you can go anywhere you want to fill it. Some optoms who want the cash may balk at this.
Yes. In most states, the record belongs to the doctor but the information in it belongs to the patient and it is neither legal nor ethical to withhold it. At the very least, the medical information in the record should be transferred to another healthcare provider asap at your request. Some states allow the doctor to charge for this but the medical information cannot be held hostage!