There were 4 undisclosed exploits on Vita’s firmware 2.61, until Japanese developer 173210 kindly released his VHBL port, “reducing” the number to 3 “yet to be revealed” exploits. But we’re back to 4, as 173210 confirmed one of his other psp usermode exploits wasn’t patched yet.
With apparently the help from Qwikrazor87, famous for being involved in pretty much every single VHBL port nowadays, 173210 was able to port VHBL to his exploit fairly quickly.
173210 shared screenshots of his work on twitter, these screenshots being taken on a PSP for now. VHBL exploits are typically found in PSP games and run on the Vita through the PSP emulator.
The typical development process involves doing most of the work on a hacked PSP, and do only the final verifications on a PS Vita.