This might be a strange solution for some people, but it worked for me so I figured I'd share it.
A lot of the avatars I rip are specifically so I can get a certain mesh, and this process is a bit tricky for me, and sometimes don't work.
I'm able to completely avoid the Blender portion of this by using this tool Fox Mesh Deleter IN Unity. It lets me delete all of the mesh I don't need right in Unity.
Note, this does NOT give you an FBX. BUT if you're not taking it out of Unity, it will still work for VRChat avatars. You can even export it as a Unitypackage to a different project.
This may be a nice solution for some people (such as myself) who aren't worried about doing edits to the mesh I wanted, just needed it separated from something else.
Hope this helps someone! ^^