Is there some kind of hidden L3/R3 feature, or one added by an external controller? Does any software use it if so (if you are looking at a computer keyboard then chances are several buttons are no longer used and you can press them all you like with no program/OS caring).
Adding some kind of switch setup to it to act as an alternative input for an existing button is one thing, indeed so basic you can probably do it with tape*, making it respond to a thing is does not know and might well be baked into the hardware and operating system** at very low levels is quite another.
*I don't know that I would over the usual find some kind of test point, or if you have to then adapt the existing traces/button pins/whatever, and run wires to whatever kind of switch, turbo input/oscillator, macro input or output from microcontroller that you like but the principle is the same really. That or the layers of tape are being used as a crude type of switch.
**most old hardware has a section of memory the games ultimately use, newer stuff, which would include the vita, tends to have the always running in the background operating system take data from the hardware and give it to the game. Also why you might be able to do control redirection at the OS level rather than needing to hack the specific game (or rewire controllers) like you might have for older systems.