Tipweight,
If it is vibration you should be able to tweek it out, as the vibration most helos put out when properly maintained is not particularly bothersome when we are expose to it for only a few hours per week. How much is too much? If it affects your speach by modulating it without any conscious effort is is clearly too high, or if it causes a noticible modulation of your vision, or if when you hold your teeth just apart, they tap together. All of these signs are indications that your helicopter probably needs to be tweeked to stop this vibration.
If the vibration is below the above, it could be a combination of posture and seat comfort. I have often seen that the reach with the right hand to ghold the cyclic extends the muscles of the lower back. I suggest that you use a small lumbar pad that you carry on, and that you retract the pedals or slide the seat forward to jack-knife your knees (so your arm must lay on your knee (thus relieving the back pressure of holding it extended.) If you still can't rest your hand on your knee, buy a clip board that fills in the gap, and rest your right fore-arm on that.
Also, I have found that many seat backs are straight right to the bottom, and should have a circular cross section pad that is a max of 2" thick that fills in the lower 6 inches with a gentle curve. This forces the spine to curve naturally forward then back as you rest against the back pad, and it also forces the whole back to recline a bit.
I had a back pad specially made for two different helos that I regularly flew and that were not right for me. I even velcrowed one to the back cushion to hold it perfectly in place.