Why is my PSVita not charging through the USB port on my PC?

I want to charge my Vita via my PC's USB port. When I connect it, it hooks up to the PC and I can transfer items back and forth, but it does not charge.

I looked around and saw a few posts saying that you have to enable it in the options, which I did but still it doesn't work.

My USB 2.0/3.0 ports work for charging since I also charge other things, such as my iPod, iPhone, etc.

Is anyone else having problems charging the Vita from a PC?

I found that it works for me, but only if I switch the system off completely (holding the power button untill the prompt comes up, then turning it off via touch). I don't have the Content Manager installed and I do have the "USB power supply" option enabled.

The USB port on my PC is a standard USB 2.0 port.

The Vita's battery is 2200 mAh, so turning it off and charging it on a USB 2.0 port (max 500 mA) would take almost 4.5 hours, which isn't great. If it's on, its power consumption rate can get up to about the same as that charging rate, so you wouldn't be doing much except turning the battery into a bomb.

According to this the Vita's charger is rated at 1500 mA, and other chargers are 2000 mA. This is significantly more than the USB 2.0 maximum or USB 3.0 max (900 mA). As Resorath says, that must just not be enough.