The ps3 dongle was a specialized usb payload that put the ps3 into a service mode. That mode was already there on the ps3, put there by sony. It is SORT OF like how the "pandora" battery put the psp into a service mode. Hardware for sony techs to use that the community learned about and could reproduce.
The vita could have something similar that we just dont know about yet, but considering just how security oriented they were when designing it, I doubt it will be so simple.
Actually, from what I understand it went like this, or is estimated to.
Sony PS3 Service Dongle was leaked/obtained by hackers Hackers researched how this device worked Hackers found that by modifying the dongle ID they could cause a buffer overflow to load their own data Hackers attempt to market this as a dongle Dongle is released, and reverse engineered by Matheiulh et al.
Sony would have to have some method to getting into service mode, something unlike their old method. Maybe it has something to do with bluetooth, wifi, a button/touch combination, or maybe a special proprietary card to go into the system. I guess only time will tell