Well, Aslong as the PSP software is gated off from the vita OS, then there should be no problem.
Theres almost no way sony could've avoided this, since PSP keys are out in the wild due to the ps3 hack. So in the eyes of the console (vita or psp) theres no way to diffirentiate between an actual game and homebrew code- assuming it was signed with the PSP keys.
EDIT:
This is actually the best of both worlds. You can have the awesome PSP homebrew stuff, but at the same time; vita is technically unhacked, you won't see any rampant piracy that weakened the PSP.
If all goes well for, it will remain unhacked for quite some time, sony have become pretty proficient at gaurding the ps3 against hacking, it was actually their own mistakes that caused it to be cracked: the PS3 debug mode, and the non-random keys. So hopefully they learned to not shoot themselves.