My C has a very slight vibration at about 60-70mph. It used to be a lot worse, but I had the wheels balanced at a MB dealership and it was improved so much I don't really notice.
The other thing I did was to buy a good digital tyre pressure guage (about 10 pounds or $15). Keep an eye on tyre pressures: makes a huge difference to straight line traking, traking under braking, handling and vibration. Checking the tyre pressures is the cheapest fix and often it the only fix needed.
Glad to hear your C280 is rewarding at high speeds. My C is good at 85-100 mph too; even my humble 2 litre 4 pot feels strong at those speeds! After all, that was the speed it was designed to operate at.