In an emergency such as that I often pick up a hammer and rest it on the shaft changing the harmonics. Works sometimes, but to leave hands free make up a roller that mounts on the toolpost so that it rests on the work could kill the vibration. Also use lots of top rake on the tool to minimise tool pressure. A chunk of lead wired to the toolpost so that it rides on the work could help. I'ts all about changing harmonics. I have lead slugs in a rubber strap that I fasten around auto rotors and drums on my brake lathes and you better not forget it or you get chatter every time. Good luck, Peter
The difficult done right away. the impossible takes a little time.