I heard rumors of people downgrading by corrupting vs0 (can someone link to the source of the rumors?). Regardless if the original poster is lying or not, there is a possibility that if you corrupt vs0, safe mode would allow you to install any FW version. If that's the case, it means you can downgrade with hardware: connect the nand to your computer, corrupt vs0, and downgrade. Of course I don't want to test this theory but if anyone wants to risk their vita for science... Use VitaRW and delete ALL of vs0 (do NOT touch os0). Let us know what happens.