Downloads:
multiBoot Menu
menus savedata

Instructions:
- Download multiBoot Menu, extract and copy MENU.PBP and DEFAULT.TXT to the exploit save, overwrite anything.
- Download the menus savedata, you'll have a folder named SAVEDATA, inside you have four folders named yMenu, pyMenu, etc, copy them to ms0:/PSP/SAVEDATA either using FTP or CMA, it's like copying any other save.
- OPTIONAL: you can edit DEFAULT.TXT to change the default menu that will be loaded
- Copy the exploited savedata with multiBoot Menu in it to the Vita.
- Start CEF
At this point you will see the text "Booting..." on the screen, if you press up or down then a menu will come up that will let you choose the menu you want, if you don't press anything then the default menu will be loaded.

To change from one menu to another just press and hold start for a few seconds, it will exit and you'll see the "Booting..." screen once again, just press up or down and select the menu you wish.

Here's some screenshots (courtesy of The Z):



When you see that, quickly press Up or Down (D-Pad) to come to this:




Source