The accessory port found only on the original PS Vita models is a custom port USB interface. The port must be activated in software and so far, there does not seem to be any official public software that uses it. It was thought that all API's that reference the accessory port were permanently disabled as of firmware 1.69, however this is not actually true: even on FW 3.73 the accessory port can be enabled using SceUsbServ#sceUsbServAccessoryActivate and disabled with SceUsbServ#sceUsbServAccessoryDeactivate.
|Ground - On the side with the mounting hole that has threads
The only documented official usage of this port is from an FCC filing for the PDEL-1001 development unit. It seems that some dongle attached to the device is used as part of their hardware testing framework.