So I finished the game and got Katherine's true ending so if the scenario changes much in other ones my apologies, but too my knowledge the fact that Catherine is a succubus is pretty much in all endings. The key to unraveling that fact that Boss is the only one that saw and spoke to Catherine. My girlfriend actually pointed this out to me but early in the game when Vincent is out to lunch with Katherine he blows off Catherine's call and she shows up at the restaurant and talks to the host/server. I don't believe it was the same voice as the Boss but that is the only way this could work without breaking the logic behind a key twist in the story. Did I miss something or did the writing staff?

you never saw catherine see the host/server

Perhaps. Maybe the server was just another guy Catherine was seducing, so he could see her as well as Vincent. 
 

@FluxWaveZ: Probably the best explanation I can fathom there. Though if that were the case I'd imagine the conversation wouldn't have been so formal.

@Junpei said:

@FluxWaveZ: Probably the best explanation I can fathom there. Though if that were the case I'd imagine the conversation wouldn't have been so formal.

could be some sort of foreplay, after all

in retrospect, I have started a new playthrough and my girlfriend was wrong, you never actually hear a server. You just hear Catherine say that they are still serving lunch and what she wanted. Crisis averted.

@Junpei: You're right, the only one you can hear is her; not any server.

That's the scene we're talking about:

... and, yes, this is my channel. (It doesn't hurt to do some adequately related self-promotion, every now and then, right? ;-P)