Unlike other forms of drug coverage, you can use either your VA drug coverage or your Medicare Part D coverage, depending on the medication or pharmacy - but you cannot use both coverage benefits for a single purchase.
Your VA coverage is also considered "creditable coverage" by Medicare and, should you decide to enroll into such a Medicare Part D plan after the close of your initial enrollment period, there will be no penalty for late enrollment.
Editorial Comment: If you are entitled to VA drug benefits - and join a Medicare Part D plan (PDP or MAPD) - you probably will have access to the most comprehensive (and affordable) prescription drug coverage available.
Please also learn more from the U.S. Department of Veterans Affairs
"VA health care and other insurance - If you have other forms of health care coverage (like a private insurance plan, Medicare, Medicaid, or TRICARE), you can use VA health care benefits along with these plans. Learn more about how VA works with other health insurance."