Teakne,
My 4106 had an engine vibration when i got it. It eventually just got worse and worse. I knew I had a "wet" cylinder, either from a cracked oil ring or a low compression. I never determined exactly what it was. Injectors can easily give a vibration and are the one of the "cheaper" items to replace.
People are right in saying that a healthy 8V71 is very smooth at all rpms. I have run my new engine on the garage floor several times to test things, and it was smooth with the original injectors in it, but even smoother with the new N65's I installed (I also advanced the timing, that might have affected things, but advancing usually makes engines run a bit rougher).
Good luck! Let us know what you find.