There is something to what challenger1 said about the vibration even if the flex plate is bent from bolt pressure if it's centered it shouldn't cause vibration.

Because you stated that the engine runs smooth at idle that would tend to eliminate any part of the rotating assembly parts being off center or heavy to one side. If the vibration increases with the engine rpm's then something has to be deflecting and possible rubbing

I run a high speed balancing machine for electric motors and it doesn't matter if it's turning 400 or 2000 rpm's the out of balance amount doesn't change the only way to change it is to add or remove weight