"I'm not sure why but I guess I had a hunch something would go wrong so prior to doing any rigging on the model I wanted to get eyes working and ensure that was all correct before doing anything. Boy am I glad I did.

I still have a lot of work to do and I now need to restart the face from scratch due to some things being welded that shouldn't have been. Let's talk about eyes in VRChat though.

Wanting to get eyes working? You need some very strange things. First, you must have a specific skeletal hierarchy. Your first bone should be one called Armature which the pelvis bone is parented to. Then Hips -> Spine -> Chest -> Neck -> Head, then the eyes (RightEye and LeftEye). I scrapped older requirements for additional bones called Eye_R and Eye_L that were seemingly never used and had no issues. For some reason you specifically have to use 2 spine bones (Spine and Chest) to make it work properly.

Whatever mesh the face is attached to then also has to be named 'Body' (which seems to be case sensitive) and has to have at least the core 4 - 5 facial morphs / shapekeys. These can be blank, they just have to exist. In case you want to use them though, they are: blink (left), blink (right), upper eyelid lowerer (left), upper eyelid lowerer (right), and following that would be a resting face (silence) and visemes which can be plugged in manually without the order really mattering.

Next, the skeletal orientation also matters which means the Hips, Spine, Chest, Neck, and Head bones have to have a fairly specific orientation.

I'm afraid I can't really provide super specific details due to the likelihood of you using a difference in program but in max the orientations look roughly like this:
https://i.imgur.com/u5XbP0P.png

Hips: -90, -90, 90 (rough orientation, not exact)
Spine - Head: 90, -90, 0 (rough orientation, not exact)
Eyes: 90, -90, 0 (rough orientation, not exact)

I don't think I'm missing anything but with some of the absurd requirements I probably am. Unity lets you plug in the eyes when you set up the rig which VRChat uses but it doesn't seem to use the eye portion of it and instead uses some whatever else.

I suppose all that comes next is a lot of manual rigging on the body and hair and a complete restart on the head."