When you sign up for Medicare, you'll become eligible to buy Medicare supplemental insurance, known as a Medigap policy. A Medigap policy pays some portions of your medical bills that Medicare doesn't pay (such as deductibles and copayments). A Medigap policy must fit into a particular plan category, which determines the types of care the policy covers, and the amounts it pays for covered care. These plan categories are regulated by the federal government.
If you also have VA health care coverage, for any given medical treatment you can choose between receiving it from within the VA health system or going outside the VA system and having it covered by Medicare. (You can't have both the VA and Medicare cover the same medical service.) Depending on what VA Priority Group you're in, you may have copayments when you use the VA health system. You'll also have deductibles and copayments when you use Medicare. The good news is that a Medigap policy can pay not only the unpaid Medicare copayments it covers but also VA copayments.
So, if you buy a Medigap policy after you enroll in Medicare, be sure to provide information about the policy to the VA medical center where you receive your care. The VA will then bill the Medigap insurance company for any copayments you're responsible for when you receive care at a VA facility.