I’m going to go out on a limb here and say that the removal of that weight for whatever reason is why he has the vibration. The flex plate is neutral balanced but it isn’t made to balance the torque converter which is now out of balance because it’s missing the weight which gave it neutral balance. If he replaces the converter I feel his vibration will cease.
This is like taking all the wheel weights off just because the engine is externally balanced and yes it makes no sense what so ever to assume that would be the case because the weight that balanced the torque converter was removed because he installed a neutral balance flex plate.