By default the control dial at Gametrix controller is disabled for USB mode, you can enable it via Gametrix Control Panel software.
Until the dial is enabled you can change the vibration strength by moving the USB slider and pressing "Apply" in the Gametrix Control Panel.
(If no luck, please try to use the Russian version of the Gametrix Control Panel to enable/disable the dial.)
If you mean Vibration Level slider in the Seat Handler, it doesn't work. I'm not sure if it ever worked.