Is there a fool-proof means of doing a 2d animated mouth for vrchat? I tried using a series of quads textured with a different mouth shape, then tied them to shape keys that toggle between either shrunken inside the head, or out in front of the default mouth, but I keep getting cases where the odd time a mouth shape key gets stuck out. 

I'm probably needing a different approach to this. I don't know if I can shape-key UV changes, for example.