I had the toothed pulleys ready (16T), I changed the flat ones, nothing has changed... Ok, the 16T aren't perfect with the motors 20T pulleys (the belts parallelism is not perfect like before), but I don't think that make a different for the VFA.
Now I'm trying to make a rubber base for the motors to try to "filtrate" the vibrations, I'll keep You informed but I'm not so confident...
Thanks to both of You to aswered me! 
@gloomyandy...
I think you got It! It's surely like you said: the issue is exactly the same, only at slow speeds and only on Y axis.
I really thank you a lot, I didn't know this issue.
@o_lampe...
You gave me a very good tip, I dismantled the carriage just arrived from China... I know how they work (I work with CNC machineries and I am Solidworks designer), there was a lot of plastic remains from the processing, I cleaned up everything and greased well. After they sun very smoothly. Next time I'll buy Hiwin stuff... 
Now remain to problem to how to solve this issues!
I read that new 0,9° StepMotors + 16T pulleys could help, but I should re-design almost all the upper part of the printer... 
Any idea?
Drivers fine tuning could help? How?
Hi to everybody! Since I had mounted the linear rails, and the Duet 2 WiFi I'm having a very annoying issue: vertical lines appeared ONLY on Y axis. X axis is totally free from It.
I don't this that is not an "acceleration ringing" because is absolutely constant trought all the vertical wall, and have always exactly the same pace of 2mm (like the belts?).
Doesn't seems an extruder's gear problem 'cause the issue pace doesn't match the gear pace, and in every case if the problem was the extruder -> the issue should be present even on X...
Those vertical lines never change even if I change speed, layer's heigh, tension of the belts, motor's set up (accelerations, jerks, Amperes etc etc).
I mounted bigger motors on X & Y (NEMA 17 48mm), nothing happened. I checked all the bearings, they are ok.
I disassembled the linear's guides carriages and cleaned up/greesed, everything move very smoothly.
Then I had the mad idea to set the printing speed to 105mm/sec and the lines totally disappeared! 
I still thinking something about the belts or the pulleys (they are "flat"), but before buy new belts or pulleys I would be sure to don't do something useless...
The machine was an Tronxy X5SA-Pro, updated with linear rails, MicroSwiss double gear Extruder + MicroSwiss All-Metal Hot-End (Direct configuration), Duet 2 WiFi (RRF v3.3.10), 7" Display.
Here is my Config.g file: config.g.txt
Thanks in advance, Otto.
P.S. - Sorry for my english but It isn't my native language...