I've been working on CC hair and I've got it working great aside from this ugly seam on the back of the neck, which only appears in game and looks fine in bodyshop. I've double checked the texture alpha and the UV mapping and both of those look correct. I also made sure to assign the vertices in the area to the head and neck %50/%50, but still no dice. I feel like I'm going crazy because every fix I look up doesn't work. I've attached the files down below. Any help is appreciated.

You've assigned the wrong vertices (although probably doesn't hurt to keep those assigned, too - they'll likely be moving along with the neck a bit better).

It needs to be the 5 bottom vertices on the "hair" group (the head part), since those are the ones that align with the ones on the neck. If you do that, it should look fine.

Oh my gosh I feel kind of silly now but that was exactly it, I must have misunderstood the wording of the tutorial I was reading :p.
Thank you!