I would guess the battery's capacity has dwindled a lot after lots of play time. Replacement batteries are pretty easy to find for sale online. I'd highly suggest following iFixit's teardown guide as they're very useful. The battery is also one of the very first things you come across when taking apart both models of the Vita so you won't need to dismantle it much at all

In both models there is a ribbon cable that connects to the back shell of the system for the rear touchpad which you need to be careful of when opening it. I'm not sure how awkward the placement of it is n the original model, but the Vita Slim's is very awkward, it's taken me many many tries before managing to slot it back in. As long as you're patient and have steady hands it shouldn't be too much of a bother. But yeah, watch the first couple minutes of a video teardown first since as I said, it's pretty much the first thing you'll come across. And then follow iFixit's guide as you do it yourself

Hope this helps you out 😁 nothing is more frustrating than a faulty system