I'm sure the board will be flooded which topics about how the PS Vita isn't charging via USB, so here is the requirements to do so!
1. Make sure you have ticked the option to turn on the ability to charge via USB in the settings menu of the Vita operating system.
2. TURN OFF THE VITA. A USB port is NOT capable of delivering power to a device like a normal AC wall outlet which is why Sony simply disabled the ability to charge while the Vita is on. The Vita uses way too much power to keep itself on and would render a simultaneous USB charge useless.
3. I've heard mixed reports about the actual power cable that goes into the PS Vita itself, but for some you can actually put the cable in backwards. Make sure the PS Vita logo on the cable is parallel to the screen itself (the screen and logo should be facing the same way, and be able to be seen together).
After completing these steps, the Vita should now be charging via USB. Congratulations!
I've had my Vita since just past midnight last night, and it charges fine while connected to the wall AC adaptor, even with the system on. I was playing around with it for almost 2 hours while it was charging. The battery icon in the upper corner started off all wobbly, and when it was done, it was a full green, solid colored battery. So it works fine with you on the system using it while it's plugged into the wall.
To the mattresses. PSN: Justice_98405, Xbox Live: Justice98405