Stan, your basic premise is just a little off - remember, it is not the number of blades that determines the total thrust, it is the solidity. All the virtues you list are attainable by controlling solidity. An additional blade is one way to get more solidity, another path is to provide the original number but with somewhat wider blades.
Regarding the vibration, the basic energy (actually the root shear forces) are very much reduced when you have more blades, and the reduction is much faster than linear, so it really helps to reduce vibration by adding blades.