Turning Vac pumps backwards....... more folklore!
This don't turn prop backwards because of the Vac pump is total and utter rubbish that is chured out by people who should know better and belongs in avations dustbin along with "don't lean the mixture below 5000ft".
The prop can be turned backwards by hand without damage to the vac pump, what you must not do is fit a Vac pump from an engine that turns to the right to a left hand turning engine (IE dont swap the vac pumps between engines on counter rotating twins).
If turning the engine backwards did as much damage as the mis-informed say we would all be changing them at the 150 hour check as checking the Mag timing requires the engine to be turned backwards to release the impulse coupling in the Mag(s).