Great news!
after recreating my project and sanitizing my avatar of props, colliders, and animation (sneaky animation broke it) I've discovered and quirk.
The VTuberplus program can handle the .vsfavatar using the 'Standard' shader, and 'VRM/MToon'. HOWEVER!
a .vrm of the same avatar using 'Standard' shader, returns `Failed to load. Make sure your avatar uses only UniVRM shaders.` Thankfully a .vrm avatar with 'VRM/MToon' loads