All you can do, is get say a 1" lump of bluetak and stick on the side of the chuck, then run it. If by moving the lump around the outside of the chuck the vibration starts to get better, then leaving the blob in position, remove half of it, if it gets worse, add some more to the original lump, by trial and error, if it is the chuck, then you should end up with it eventually being balanced.
Then it is a matter of removing metal from the backplate or chuck, diagonally opposite from where the stuck on weight is, equal to the weight of the lump.
I balance critical flywheels and rotors, but that is easy because I can mount them on a free running spindle, where the heavy bit always drops to the bottom. Unfortunately, you can't do that with a chuck.
You could try to rotate the chuck around to the next set of mounting holes, and see if that balances it out.
But other than that I can't suggest any other way of doing it.
Bogs