Your clutch bearing has seized and now the entire bearing assembly is spinning in the clutch casing. Common problem on that model.
The amount of parts that will need replacing depends on how much you've used it since the vibration started. At best you will have to replace the front cover/clutch casing, which includes the clutch drum and bearing. At worst, I've seen units where the clutch broke apart and destroyed everything on the front of the engine, making the power head a total loss. There's really no way to tell till you get it apart.
How much mechanical experience do you have? You'll need to remove the green casing to see what's happened. Not a very difficult job, but you'll have to remove a decent amount of parts to get it off...