[Eexhaton]

The extent of vibration is going to vary. The Merlin fior instance was generally declared to be a sweet running engine (even by the Russians of the time) whereas some radials were much more rough. Also, there would be some variation for the same engine (perhaps too fine to be noticed in terms of game feedback) due to manufacturing differences. I would imagine an end-of-war DB605 might not run quite as well as one built a year earlier. Maintenance is going to make a difference too.

Although I don't have personal experience of WW2 era aero engines, I remember the difference between the installations in the light aircraft I flew. They all felt different, even if the sound wasn't so varied. The Gypsy Major of the DH Chipmunk seemed lumpy but eager. The C150 was cantakerous to begin with and a little coarse in-flight, but I remember flying the Beagle Pup and thinking what a smooth aeroplane it was, not just because of the higher wing loading, but because the engine felt so much better balanced.

Ofcourse, you are right.

Still, it's about the technical side of actually getting a vibration-effect ingame.

If anyone can get that done in the 1st place, then it's on about how severe each engine/airframe combination would vibrate if that is even possible to configure.
One of the ideas I'm having on this, would be to apply the forces of the different aircraft guns. for example 20mm cannons vibrate differently then a whole bank of .303 machineguns in IL2. This effect could be used to simulate different engine type vibrations.