I believe you will find the centrifical force exerted above 60 MPH will "hide" slight imperfections in tire balancing, tire roundness, and even wheels that are not true. But you can be sure your tires are suffering at the higher speeds, because you haven't really "fixed" the problem. Besides, 50-60MPH is a fairly common speed and you will surely feel the vibration at that speed. The suggestion to check thoroughly for a bent wheel is really your best bet.
If you are really bugged by the vibration and can't find an obvious cause such as a bent wheel, find a place that balances the front tires WHILE ON THE CAR. Some of the Shell repair facilities can do it--requires a special machine, not like the units that spin balance the tire while off the car. By balancing tires on the car, you can compensate to some degree for balance imperfections in the wheel and rotor. The down side is if you ever remove the wheel for any reason, you need to mark the wheel in relation to the lug bolt and replace it in the exact same hole to preserve your balance.