Varis Zos Galvus isn't an idiot by any account, though. He's described as a great tactician so I am fairly certain he would recognise his own son and realise if anything was off if someone - especially a woman - tried to impersonate him. They already pulled that trick with Nael and again with Lyse. Pulling it for a third time would just be silly - and Zenos is already confirmed as being the main antagonist of Stormblood. He was also described as someone people wouldn't sympathise with at all. It's likely he's just someone who is rotten and opportunistic, caring little for anyone but his own amusement. That certainly was the impression he gave upon hearing the report of the damage done to Gyr Abania.