Vulkan is not supported on these devices!
Unfortunately, removing the Vulkan entry from the backend options isn’t possible in the PPSSPP emulator. So if you saw the option and felt inclined to test it, you might now find yourself unable to launch any PSP game—or even the PPSSPP app itself.
The easiest way to revert back to OpenGL is to navigate to /MUOS/emulator/ppsspp/.config/ppsspp/PSP/SYSTEM on sd1 and delete the FailedBackends.txt file. After that, PPSSPP should launch as usual.
If you’re doing this directly on the device, you can use the Dingux Commander app. In that case, the file path will be:
/mnt/mmc/MUOS/emulator/ppsspp/.config/ppsspp/PSP/SYSTEM