Size I will be interested to have you ride my Roadster sometime for you to compare the vibration in mine to yours. I just did a 2000km ride, the bike now done 9000km, the ride gave me plenty of time check the vibration, it comes in at 2100rpm, disappears at 2500rpm and come back at 2700rpm and stays in to 3500 rpm, on and of the throttle, and just like you say it completely disappears with the slightest pressure on the clutch lever.
It not an unbearable condition but would great if it was not there, the bike would be perfect.
I did speak to Triumph, they told me there were aware of the condition but not considered to be detrimental. I will have the bike shop have ride when the 10000km service is done and give their opinion.
By the way are you still riding your Rocket while waiting for your new one?