Minimal user interfaces are all the rage these days, though the PlayStation Vita may have taken it a step too far by neglecting to add a percentage option to check its battery level. If you're insanely curious about the amount of juice left in your handheld, well, there is a way to get a reading... it's just not as practical as we'd like.
PlayStation Lifestyle writes that to check the battery, all you need is the Vita's USB cable and a PC. The instructions are very straightforward: Plug it in, skip the Content Manager Assistant and bring up the Properties box for the handheld — once it is detected and available in "My Computer". You can access the Properties dialog by right-clicking on the PS Vita's icon or highlighting it and pressing Alt-Enter. This will also show you the Vita's firmware version.
While not the most portable way of going about things, it could come in handy for diagnosing a faulty battery or device, or seeing how much power a particular game or activity drains. Hopefully a more practical method will present itself in the future.
How To Check the Vita’s Battery Percentage With a PC [PlayStation Lifestyle]