There is a USB client on handheld Vita devices. It uses a proprietary multi-connector on the OLED (PCH-1XXX) units and a micro-USB on the LCD (PCH-2XXX) units.
Because of the existence of the USB ethernet adaptor, that means there exists a way to use the port in host mode probably through USB OTG or something similar.
Teardown of Vita OLED dock here.
|1||Power?||No known use (N/C on cables, USB ethernet), goes to SN99057 pins 31-32|
|2||Power?||Same net as pin 1|
|3||Analog Ground (TRRS Pin 3)||For stereo output|
|4||Right Channel Audio (TRRS Pin 2)||To audio codec IC|
|5||Left Channel Audio (TRRS Pin 1)||To audio codec IC|
|6||UART RX||See UART Console|
|7||UART TX||See UART Console|
|8||UART CTS||See UART Console|
|9||UART RTS||See UART Console|
|10||Audio?||To audio codec IC, not seen used anywhere.|
|11||Accessory Enable?||Digital signal. Pull-down to GND on dock. To Syscon (C11), connected on USB Ethernet|
|12||TRRS Pin 4||Digital signal. To Syscon (F9), connected on USB Ethernet|
|13||?||To Syscon (G4), not seen used anywhere.|
|14||GND 1||GND on Vita and USB ethernet adapter, N/C on charging cable and dock|
|15||?Peripheral Detect?||To large ceramic chip, 125K pull-down|
|16||GND 2||GND on Vita, N/C on USB ethernet and charging cable, small cap to GND on dock|
|18||VCC||Same net as pin 17|
|19||GND 3||GND on Vita and charging cable, N/C on USB ethernet, large cap to GND on dock|
Port is a standard micro-USB Type B. Highly likely that USB OTG is supported in hardware although there's no evidence that it's enabled in software.