Because the Vita was less popular everywhere outside of Japan, which makes that there are less units available and the units cost a few times the price of a psp second hand. (in The Netherlands the PSP is just under 100 Euro second hand, usually with 20+ game discs, a vita starts at 200 without any games and often without a charger)
A lot of people also still have a PSP just hanging around, less so with the vita.
A lot of what Proton-GE brings from my understanding is a larger support for Media Foundation, which can't be added to Proton itself because of license issues (Proton is from a commercial company, where Proton-GE is from an individual).
So aside from the stuff that has been implemented differently, running Proton instead of Proton GE is like trying to game on Windows N editions.
I played the F-Zero game recently in an Arcade nearby, it was amazing! I was so suprised when a buddy of mine went like: "Yeah, there is just a Gamecube in this".
It was very broken for a long time. Since fairly recently you have WiVRn (specifically wivrn-dashboard on Arch) for Oculus (more supported though) and I would daresay it works better then SteamVR used to do for me on Windows
There is a chance the led is also used as a important diode in the circuit, plainly removing it can greatly reduce the lifespan of the device. (more common in cheaper products)
Adding a appropiate diode in it's place is advised.