When you have a good thing, you want it to last. To extend the life of your Yokohamas, they should be rotated every 9,500 – 13,000 miles. If you have any unusual tire wear, performance, or ride quality, please take your car to a professional mechanic or retailer.
A vibration when driving could be an indication there is a problem with the balancing of your tires. Especially if you feel a vibration that is dependent on vehicle speed and first becomes apparent between 64 and 72 miles per hour.
For more details on mounting, rotation, balancing, and suspension, check out Advanced Tires 101.