If im wrong please correct me
As i understand this the reason why this happens is because the specification minimum for usb 2.0 is to supply 100mA up to a max of 500mA (i believe this is the standard). There are usb 2.0 ports that can supply more then the standard 500mA, these are usually called charging ports. they can supply an undetermined max (i dont think there is a standard max for charging ports). On some PC motherboard they have a funny name like for example gigabyte calls it on/off charge (they also call it 3x usb power) On gigabyte boards this supplies up to the 1500mA power. Coincidentally the ps vita is also a 1500mA charger so this should be enough to charge it as well as the wall charger. I have the gigabyte board i am talking about with the usb 2.0 3x charging ports but i havent tried them yet (been meaning to just havent had the time yet)
What about usb 3.0 you ask?
well usb 3.0 standards are i think 150mA minimum up to a maximum of 900mA. is 900mA enough to charge the vita i have no idea and if it is enough for the firmware to allow charging i can guarantee you that it will be much sloer then using your wall charger. Thankfully alot of manufacturers (well as far as ive seen anyway) have implemented at least 1 charging port on mobos and especially notebooks. (my mobo is an x58 socket 1366 board so its not too new)
Usually a charging port is colored differently from the other lame regular ports but the easiest way to tell if you have one is to look in the documentation for your product (ie mobo or notebook manual)
The reason why shorting those wires works is because thats how you are supposed to tell the usb circuitry that the port it has just plugged into is a charging port. Although this works be careful when you do this as from what i remember (dont quote me here as this is a little of a blank lol) there isnt a maximum draw specified that the device can take. If your wiring or power supply cant take it (aka the battery) you could damage something.
Hopefully all this information is correct as im not 100% on it. Maybe at least this will inspire you to look it up on your own and prove me a fool