If anything I think you may have the 20x10 Multi with the 25mm in the rear.
That probably could explain the 255/30 rear tire on the wheel cause that is a tiny tire for a 10" wide wheel. Also if that is the case that 25mm I think has a 74mm centerbore which was for the E39 5 series.
That could be the reason for the rear vibration and a simple hubring should do the trick. But yes of course exact information would be needed.