If you shove the clutch in at vibration speed and you still have the exact same vibration its not in the engine or flywheel. When you shoved in the clutch you disengaged the the engine and flywheel from the trans and drive shaft and rear end, if you still have the vibration its in the trans or drive shaft/rear end. Put the rear end on jack stands with rear wheels off the ground and see if you can duplicate the vibration, if you can't duplicate it on the stands it's not in the drive train. Try the tires and wheels.