I would always suggest turning off the fan when doing high magnification.
Why would you turn it off? On an excellent night, splitting double stars at the Dawes limit, I am at 821x with my 10 inch F/5, I see no vibration, I see clean diffraction rings.
The fan does not vibrate, I leave it on...
Is your 10 inch F/5 providing clean, sharp stars that rival a high quality apo at similar exit pupils? This can happen if you pay attention to thermal issues.