Control a USB light via software
I don't think it is possible, because the power (Vcc) line on USB is always-on--it is not intended to be a control line.
Unless your USB light already provides a way to control itself via USB data, you are out of luck. I do not recommend trying to change the voltage of the USB power line.
Your best bet is probably to buy something like an FTDI interface cable, available from Digi-Key for $20. That gives you +5, ground, and four wires you can switch high or low under software control. The software's easiest if you use the thing as a logic-level serial port with rx/tx/rts/cts. The manufacturer part number is TTL-232R-5V-WE (go to digikey.com and type that number into the part search box). Digi-Key has links to the manufacturer data sheets and web site.
Maybe you can tell the USB controller to cut off power to the USB port. I've seen this option on the power saving settings on my PC.