I had this problem as well when I got the car, but I had bad tyres and bent rims. After I got the new tyres I still had a constant vibration only from about 55 to about 65 mph, so then I decided to get some new rims. Now I got the new rims as well and only sometimes I still have a slight vibration when going at that same speed, and sometimes I have a pretty bad vibration when braking. Next thing I'll try to do is to get some new brakes... When I got the first wheel alignment check I also found out I needed new track control arms so I got them as well.