The accessory port found only on the original 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 as of firmware 1.69, all APIs that reference the accessory port are permanently disabled. however this isnt true, even on 3.73 the accessory port can be enabled using sceUsbServAccessoryActivate and disabled with sceUsbServAccessoryDeactivate
The only documented 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.