On many of my cars, when I felt a vibration coming from the engine, it was worn out motor mounts.
If you had a spark plug not firing correctly, you would have more than a vibration problem, but more of a strong shaking.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".