PlayStation Vita does not use a traditional "BIOS" file in the same way older consoles (like the PS1 or PS2) do. Instead, the console and its emulators rely on System Firmware System Font packages to function. How to Obtain PS Vita Firmware If you are setting up the Vita3K Emulator

PSX BIOS – PlayStation PS1 BIOS: How to Download and Install

Official Firmware File:

PSVUPDAT.PUP (Main system firmware).

What You Actually Need (Instead of a BIOS)