Well, we still did not solve why IN UNIVERSE he is called Vader.
I don't suppose Sidious called Ani Vader cause "Hmmm... Vader means father, cause you make me think about my daddy"
One suggestion would be Vader as in "invader" like Sidious as in "insidious" but obviously you don't add in- in frount of all Darth- sith names.
Think about inbane, inkrayt, inmalek or inbinks. They all sound stupid.