When I was experimenting with the V-USB library I found it useful to monitor the message being sent in to the computer an compare them with a traditional USB keyboard. The best tool I found for this on the Windows platform is the HHD's Free USB Analyser.

One word of warning in the installer can pull in some "crapware" that was caught by the virus scanner but I called the company and the site (separate from there main site) is apparently legitimate.

The reason why I mention this now is that I have had some problems with Virtual Box and VMWare USB integration and tracked it down to the analyser. Uninstalling it fixed the problem and you can reinstall without issue when it is needed again though.

Happy debugging.