Once run in the motor is extremely smoothe! It will also depend on your muffler/pipe a little wether it will be super smoothe or not though. In the beginning you might have some vibrations but it will be because of the very rich running motor during the first 3 gallons or so. After that it doesn't vibrate any more than other motors. The YS has been reported to be a little smoother than the OS BUT you are not going to get quite the same amount of horses out of the YS. Stick with the OS and just make sure you keep an eye on things the first 3 gallons with the very rich settings as you'll have some vibrations then. IMHO though if you're worried about a little engine vibration shaking the heli apart I'm not too sure I want to be near that heli when it spins up?
