Tire Tread/belt problems of the 1999 Audi A6

Events: significant vibration from right rear of car at high speeds , Audi A6 at speeds above 45 mph. Failure: 2 of 4 futura 2000 tires on car showed what service manager described as belt slippage or separation. One tire was visibly oval in shape when turned on dealer balancing apparatus and second had abnormal wear patterns. Other 2 tires appeared okay. Set of 4 tires were purchased at same time in mid 2005 and were used approximately 50000 miles , 70000 mile tire, purchased 4 new tires. Dealer kept 2 defectives and I kept 2 better tires.