EHCI

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 "CBPS" was able to get the accessory port working and able to mount a USB drive via VitaShell by hacking Mini-USB cables.

From FCC filing for AK8PDE1001A

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.