Yep,
For the 9mm belts there are kevlar cored options which work well, I’ve not seen any source for 15mm kevlar belts though and the 15mm fiberglass are similar to the 9mm steel / kevlar core belts.
As for the vibration, there are several things at play.
The double belts only provide double holding force when the Z carriage is in the center of the X beam, at either end you are back to a single belt worth of force.
There is a substantial difference in moving mass for an X vs. Y deflection, more mass tends to resist vibration.
There are also complex flex modes, particularly the Z carriage nodding around the X beam which get excited differently based on the force direction coming from the cutter.
More here
That’s about the SO3 but most of the same things apply to the SO4