The vibration is caused by the impact of the mirror slapping into the frame of the body when you release the shutter. It is present in the Hassy, obvious in a Pentax 67, and non-existent in a Rolleiflex (or any other TLR). It is most problematic at the slow end of the hand-holdable shutter speed continuum- between say 1 second and 1/15th, maybe up to 1/30th. Faster than that, there isn't enough time for the vibrations to record. Slower, and they fail to record as well because they don't last long enough. Oddly, the lack of sharpness they cause will be more noticeable when the camera is on a tripod. Hand-held, your hand is more likely to induce softness by its inherent instability than the mirror slap is.