I do know the ESB Vader stunt helmet has little windows cut into the face mask for that reason.
Yeah, it had see through cheeks and they usually kept the metal screens out of the mouth holes. Supposedly that’s also why the chin vent was made so much bigger in ESB - every time you see Vader cock his head back while talking to someone, it’s because Prowse is tilting the mask up so he can see the other actor through the chin vent.