Do you have some sense where the vibration is coming from? ( front / rear or one side ) Have you revved the motor to say 3000 RPM in neutral?

With the wheels on the ground, ( or at least on ramps. ), motor off, trans in neutral, grab the axle shafts and move them front to back / up and down to check for play. They will move left to right so don't confuse that with other looseness.

Does you car have a tripod inner joint? What can happen is the lobes wear then the shaft no longer runs concentric with the joint casing.

The problem won't be a release bearing or warped flywheel. There is a ever so small chance the clutch is oil soaked / pressure plate spring weak allowing the clutch to slip / stick but, the engine will flare up when this occurs. If you have a dual mass flywheel it could be slipping but the engine will flare up if you get in to the throttle hard.