A USB device can only use a SYS driver. It cannot use a VxD driver. The reason for this is that USB devices must use WDM (Windows Driver Model) drivers, and WDM drivers should be implemented as SYS drivers. Therefore, windrvr6.vxd (or windrvr.vxd — in version 5.2.2 and earlier) does not support USB, and you should use the windrvr6.sysdriver file instead (windrvr.sys in version 5.2.2 and earlier).
NOTE: Beginning with version 6.2.0 of WinDriver, VxD drivers are no longer supported for any of the buses supported by WinDriver (including PCI/ISA).