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 adapter, that means there exists a way to use the port in host mode probably through USB OTG or something similar.
Port is a standard USB micro. Highly likely that USB OTG is supported in hardware although there's no evidence that it's enabled in software.