You can never judge anything from such material. I do know that Herbie Hancock is using an Oberheim to modulate his (Sennheiser) Vocoder with. I see Oberheims but couldn't detect the Vocoder - but that means nothing. But I can see that the notes I hear from Hancock is not what he is playing.
Stevie is certainly singing live, because you can hear the soundengineer in trouble.
If they are faking for the most parts, I would understand - to do this live in 1985 is tough.