1: Plug your vita into your PC via the charging/USB cable.
2: Press the Windows key + R, in the search box type System32 (or navigate to it manually "C:\Windows\System32" or something)
3: This is all the Vita protection files, delete the entire folder. Sony tries to hide them quite well on your own PC, but not well enough.
4: Your Vita can now play any ISOs and homebrew that you place on it.