Yi Fan Lu has posted some news concerning his continueing Vita Hacking attempts:

For the last couple of months, Iíve been developing an HDMI mod for the Vita on my free time. I thought it would be a fun project to practice my hardware design skills even though the end product would not be too useful (the VitaTV already exists). Unfortunately, this project did not end in success but I want to write about it anyways so you can see what Iíve been doing with some of the leftover money from my adapter project.

The Vitaís SoC (named Kermit) has two MIPI DSI output ports. On OLED units, the first port is connected to a custom 40-pin high speed board-to-board connector that mates with an AMS495QA01 OLED panel. On LCD units, the same port goes to a ZIF connector. The second port is unused on handheld Vitas and is connected to an ADV7533 on the PSTV. On development kits, both ports are used (one to OLED and another to ADV7533) and I suspect thatís why the SoC has two ports in the first place. I would like to comment here that the Kermit SoC does not have native support for HDMI/TMDS signaling and therefore any rumors of handheld Vita consoles having HDMI output capabilities are false. No, that ďmystery portĒ does not have video output capabilities (it is a USB host port with a custom physical connector).
Can we hook up the unused MIPI DSI port? Unfortunately no because those pins are not routed so it is impossible to get to them, so instead the idea is to ďhijackĒ the DSI output to the OLED panel and let the same signals drive a custom board that can convert it to HDMI. This requires us to solder some wires to the video signals and thanks to the OLED datasheet along with some connectivity tests, it was easy to locate test points for the desired signals.

more here http://yifan.lu/2017/12/31/vita-hdmi-mod-attempt/