Never come across these devices before but I would suggest you consider changing your graphics card to one which outputs to two monitors. This is the standard way of achieving display on two monitors.
I suspect you will find that the bandwidth of USB2 will give a poor result for outputting to a VGA display. Assuming a fairly standard screen resolution of 1280x1024 with a refresh rate of 60Hz and assuming 6 bits per pixel would correspond to over 470 Mb/sec. In real life applications, I would not expect USB2 to be able to sustain anywhere near this data rate. These are just my quick thoughts, I have not thoroughly researched this subject.