I’ve discovered something quite interesting while browsing Bambu Lab web site:
Look at the pulleys, they are toothed. 
So that mean that they have designed the axis with toothed pulleys, but for a still unknown reason they have installed the slick one.
The logic would be that we use toothed pulleys on the toothed side of the belts, and slick one on the slick side.
I’m convinced that tooth on slick pulleys create some vibrations that are transferred to the print that’s just aside.
I’ve to agree that’s strange the some of us suffer from huge VFA and some other only have light VFA.
But, in my case, the fact is that I’ve had VFA on my X1C since day 1 ONLY on X axis, and I wasn’t able to get rid of them with just belts tension, I spend hours trying different tensions.
So why only on X axis and nothing on Y axis ?
I’ve first changed the belts for Gates 2GT, no noticeable improvement.
Then I’ve installed the motor dampers, no improvements.
Finally I’ve exchanged the pulleys → NO MORE VFA !
Before and after on my printer :
And since nearly no more noticeable VFA :
And I’m not the only one obtaining such good results, some others (who didn’t post their results here) changed the pulleys after having seen my results, and they also have lot less VFA after.
Hard to believe we are just lucky.
Most probably the toothed pulleys aren’t the Graal that solve by themselves the VFA, but I’m personally convinced they are part of the solution.