Setting aside that Vorbis being any kind of anything other than Vorbis guts the themes of the book... Vorbis never lost any kind of conciousness or functionality when he left the Citadel and crossed the sea to threaten the Tyrant. It was only after he was sunk and left for dead - except by Brutha - that he seemed to take a shock and go mute. If he was dependent on worship, he should have shown signs of starting to fail while he was still in Ephebe. (Om also doesn't show signs of regressing into turtle-hood too much when Brutha makes it out of the desert and back to the Citadel before he does, but since all he could focus on was getting back to Brutha, and the distance wasn't vast, I think we can accept that as a light exception.)
Also, gods don't usally die in the messy mortal fashion, they just lose whatever body they inhabit and dwindle into nothingness afterwards without worshippers (@Valorum has pointed out in comment they do die, just not like humans). That's really what happened to all the nameless small gods in the desert. Given that Vorbis took a turtle to the face at terminal velocity (all puns intended) and definitely died and immediately passed on to the afterlife (Death being an expert on when this should happen), we can take this to mean he is definitely not any kind of god.