Jabowacky
The mixture ICO function is ubiquitous in carburated aircraft engines for a reason. The manufacturers decided it is the best way to shut down the engine. All of the Lycoming and Continental engine manuals say to shut down the engine with the mixture as do every POH I have ever seen. I have yet to hear a compelling argument to do otherwise
As for the C 421 engine vibration question. Well this engine has a vibration dampener disk as part of the starter drive assembly. At cruise RPM it is spinning at over 10,000 RPM so if its bearing start to fail and it starts wobbling you will get a high frequency vibration. A total bearing failure will quickly result in catastrophic vibration which could result in the engine coming apart. Ignition or fuel issues will normally result in an intermittent and/or variable vibration, but the sudden onset of a steady high frequency vibration should result in an immediate landing at the nearest suitable airport. A vibration that is increasing in intensity should be cause to shut down the engine.
The point of my question is note the importance of a thorough understanding of the engine and its associated systems and that thinking all of us are Luddites who mindlessly follow OWT is to discount the experience of posters who have flight time measured in multiple thousands of hours and maybe have learned a thing or two in those hours.......