I think it would greatly increase the penetration of managed care. If people are given vouchers for medical insurance they would take those vouchers to insurers and presumably be put into those company's Medicare Advantage plans which are managed care plans. The actual sector of Medicare Advantage might go away or be renamed but I suspect it would all function about the same as they do now.
For independent primary care docs this could be devastating. If all your Medicare patients suddenly have to join these plans and are told that they must now switch to in network doctors you would have no choice but to contract with the plan and thus you'd be bargaining at a big disadvantage.