Wall of text... My apologies
Please don't worry, you are the only one put out by your Vita not working. Take all the time you need to respond.
Sorry that the software solution ideas that I had didn't solve it... I want to state that this doesn't for sure mean the issue isn't software - just unlikely.
If you want to be sure it's not plugins:
If you want, you can copy and paste the contents of ur0:tai/config.txt and ux0:tai/config.txt to this thread so I can look through your plugins. Since your controls don't work, you can do this by opening VitaShell by touch and hopefully you can hit select to connect the vita to a computer via FTP or USB to access ur0 and ux0.
Probably not plugins though...
If hardware, diagnosis can be challenging... If you choose to go in (disect):
A Phillips precision screwdriver can get you in. There are screws on the back, 2 screws on the bottom and 1-2 screws on the top behind a cover (1 for SLIM and 2 for OLED). After getting all 7 or 8 screws out, I like to pull using a fingernail behind the L or R button. Open from the top like a clamshell - there are belts connected at the bottom.
I'm guessing your Left controller board or it's belt is cracked or corroded because sometimes it works. But I'm guessing.
If nobody has ever opened your Vita
The issue could be corrosion (humidity or water damage):
Belt connectors corroded.
Motherboard belt connector corroded.
Controller board belt connector corroded.
If it's corroded, you can probably see evidence of it on other board components. Opening the vita and looking at where components are soldered to the board may show evidence - if they appear to have a residue on them and next to them (look dusty white, brown, red or dark). You could look at the belt connectors themselves to see if they look shiny.
If corroded, you can just give the connections a good scrubbing with a soft toothbrush and rubbing (isopropyl) alcohol high percentage. Be Very Very gentle with the connectors on the boards. Make sure it's dry (wait) before reassembly.
While looking for corrosion, look to see if your belts are damaged. There is a main belt to each controller board. Copper colored and wide (on a SLIM, one goes behind the battery and also connects to the power button).
Another could-be issue is cracking the controller board. Accidently pressuring the sticks hard or buttons really hard can put that pressure into the board. Not able to check this visually unfortunately. Rule out corrosion and damaged belts - and then try replacing boards.
Finally... It could be (unlikely but possible) a broken motherboard. Something may have happened to overvolt or overheat a chip or trace that controls the controller board.
Belts are very inexpensive, like $2.00USD.
Controller boards are pretty inexpensive. I've received 1 out of 10 that didn't work... so I suggest buying 2 at a time. $5-$10USD.
Motherboards are about $40USD. Motherboards are always used... Out of 20+ motherboards that I have bought, only 1 had a broken feature (3G board with nonworking 3G port).
Replacing EVERYTHING without diagnosis for $55 USD I'm pretty sure it would work again.
If you need links for parts on Ali-Express, lete know.
Another option:
Sell it as a "for parts" vita before you put time, effort and money into it - and let someone else fix it. Probably sells for $80USD "as-is for parts" without charger. Buy another fully working Excellent condition Vita for $150USD without charger.
Sorry that everything is in USD, I don't know how much this translates to.
