My experience has been that the Century class ships (at least the Century) vibrate more than the "M" class ships. We were in a Century aft balcony cabin when the ship was leaving port and the vibrations would shake your fillings lose! On the other hand, we just spent a week in a aft cabin on the Millie and the entire cruise was very smooth. We never experienced any vibrations when leaving or arriving in ports. In fact we fell asleep and missed the sailaway at DR, and at St Thomas we woke up and looked out our balcony and their was the bow of the Sun Princess larger than life right next to us. We never heard or felt vibration.
The "pods" on the M-class reduces vibrations, while the traditional fixed props on the Century result in a more noticeable level of vibrations. Either way the vibrations are part of the overall cruise experience. I'll be shaking on the Century in six days!