Not having a mirror does reduce vibration, but even more important, not having a mirror makes it possible to use non-retrofocus normal and wide lenses, because the lens can be closer to the film, so it's easier to design a better lens for a rangefinder camera (at least wide and normal lenses--that's why there are so many excellent lenses in the 85-100mm range for 35mm SLRs) than for an SLR.