You're looking for information on downloading and verifying the PS Vita firmware 3.73.
: Many enthusiasts prefer 3.73 because it was the final version before Sony implemented stricter 2-step verification and device password requirements for PSN logins in version 3.74. Broad Compatibility
PS Vita Firmware 3.73: A Guide to Safe Downloading and Verification
If you own a PS Vita and are looking for Firmware 3.73, it’s important to understand what this version is, where to get it safely, and how to verify the file’s integrity.
Improved system performance; last version before major account security changes. 3.74
Alternative: Should You Update to 3.73 at All?
Before you proceed, consider this: If your Vita is already on 3.60 or 3.65 and you’ve installed Enso (permanent CFW), do NOT update to 3.73. Those firmwares are superior for homebrew.