SFM

If forced vibration is what you have, confirming the integrity of the machine tool and its power train is a critical first step. Reducing the feed per revolution is one way to determine if it is the variation in the cutting process that is forcing the vibration. Changing the SFM or rpm’s by at least 25% is also something to try (increase or decrease!)
To eliminate self excited chatter decrease the length of the tool in the cut, and substitute more rigid tooling and support materials (a carbide boring bar deflects less and can make three or more times heavier a cut than one made of steel for example) .