Pro Team's PSP Custom Firmware for the Vita. ARK can be downloaded here
I've made a bunch of direct boot homebrew bubbles using Hackinformer's "Bubble Maker" process. The only issue I have with these types of bubbles is that they are not usable from within Adrenaline and sometimes seem to be incompatible with some emulators/homebrew. For example, I could only get a certain slightly outdated version of SMS+ and gpSP. I want to have bubbles that use the old school PSP eboot format instead of pboots so that I can choose whether I open the brew in live arena OR Adrenaline. Pboots are "corrupted" in Adrenaline and not able to be launched. Plus they just make my XMB/Game folder a MESS. Not sure if that makes sense how I described it.