Vanderbilt University Medical Center and BlueCross BlueShield of Tennessee are at a stalemate over whether the hospital system will be covered in the 2016 Medicare Advantage network. Both the health system and insurer have sent letters to Medicare Advantage patients in recent weeks warning that people may have to switch doctors to keep BCBST, or switch insurers to keep Vanderbilt physicians. Medicare Advantage is a health insurance program that substitutes for Medicare Parts A and B benefits for people ages 65 and older. Letters, sent to The Tennessean by two patients, were both dated Sept. 28 and point to a disagreement over rates for the 2016 calendar year.