If this is the first time you install RetroArch you should be set. Install the Retroarch.vpk file and you're set.
However there are many report that a clean transition from the standalones to the the AiO VPK is not currently possible. The transition however should be rather easy if you follow these steps:
*Use VitaShell to move your uxo:data/retroarch/ over to your computer then delete the folder on your PS Vita
*Install the retroarch.vpk file on your Vita
*Open the newly installed AiO RetroArch at least once so it can create the new uxo:data/retroarch/
*Finally, use VitaShell to move over the system, savefiles and savestates folder from your old retroarch folder into the new one and you should be set!
*Don't forget to use the Load Core option prior to loading any ROM. Contrary to the PC and Android versions, RetroArch still doesn't detect the Core based on roms... yet. -Wololo
THIS IS THE 10/12/2016 EDITION