RE: vibration causing rpm loss?
Good advice.
The slight variance in tach readings you are seeing is probably a mixture of things, including the tach as described.
Twins setup in sidewinder mode do seem to also have some strange tendencies.
I have a P-61 with twin Saito 125's in sidewinder mode, and although set correctly on the ground the right engine leans out in the air.
There is also an rpm setting where the is harmonic addition of vibration.
I have read some articles warning against sidewinder mounting due to these problems but mine flies ok, and I can cope with the slight problems, and the vibration is not damaging, so ... I fly it and enjoy it.