Copy the Undertale game folder to your Vita's ux0:app/ directory via VitaShell using USB or FTP.
Undertale was officially released for the PlayStation Vita in 2017. To run the game using the format on a modded console, you must use the NoNpDrm plugin which bypasses digital rights management by using fake licenses. 🛠️ Setup Requirements Undertale PS VITA (USA) (NoNpDrm)
In VitaShell, go to the home screen, press Triangle , and select Refresh LiveArea . Copy the Undertale game folder to your Vita's