Have you taken power cards/Peak pressures to make sure the engine power output is balanced?
Have you checked the coupling/vibration damper between the engine & the alternator?
Alternator bearings, are they Ball/Roller or plain?
Is there any electrical problem? unbalanced amps between phases? Weird Power Factors?
Did you discover this vibration manually or by taking vibration readings for analysis?
Could the Rotor windings be moving in their slots?
What sort of frequency is the vibration? (Linked to 60Hz/3,600 RPM which would indicate an Electrical problem, or 900 RPM which would indicate a mechanical problem on the engine, or 450 RPM (for a 4 stroke engine) which would indicate imbalanced cylinder power outputs from the engine.
Lots of questions, but we need background information to work on.
It is always better to ask a stupid question than to do a stupid thing.