Hey, I found the part number. It's 73711FA000, superseding 73710FA020. I'm not sure what it does with the vacuum, initially I thought it was part of the pulsed secondary air injection system to clean up exhaust emissions, but that one looks has a little filter hat on it. Here it is on the official website - https://parts.subaru.com/p/49283327/73711FA000.html
Disregard below, I finally found an EJ18 vacuum diagram and it's listed there. A friend and I suspect it's similar to the IAC, it supplies extra air for the engine to rev up, so it doesn't bog when the A/C compressor is being run.
For some reason it's listed as part of the air conditioning system and part of the A/C diagram, despite being connected to the intake manifold. It's not listed on any of the vacuum routing diagrams as far as I've seen, which is why I think so many have had issues finding it.