NOTE: Modifying a PlayStation Vita or installing unofficial firmware can void warranties and risk bricking your device. This post focuses on obtaining official system update files (PSP2UPDAT.PUP) and installing them safely.
For the Vita, PSP2UPDAT.PUP is the master firmware file that contains the entire operating system for the device. When you perform a standard update via Wi-Fi, your console downloads this file from Sony's servers. The system then processes the file through the updater located at ud0:PSP2UPDATE/ , unpacks it, and flashes the new firmware to the system. psp2updatpup new download