This is a very interesting idea actually.
Depends on the engine of course as to how big the benefit is
and what the driving conditions are. I just drive around town
up and down hills, so i would expect an increased benefit
from this rather than a decreased benefit.
Vibrations - if the lubrication system in a engine is working
really well then you wouldn't expect to get more vibration.
Actually, you might expect less. No explosion/combustion is
more likely to make the engine run smoother.
At lights and when the vehicle isn't moving, one doesn't
need more power. Being stuck at lights is a total fuel
waste.
This system could be improved by hooking the switch so
that it automatically engages after a certain engine
temperature and when the car is in neutral gear.
so for me it is
