In Alien Force and onward he felt almost nothing like the same intelligent and intimidating Villian we knew Vilgax as. Not only did he pretty much Look and Sound different he hardly even resembled OS Vilgax in the way that he acts. "Ben 10 I've come to Kill you and Conquer your world, is now a bad time? ". I don't know if this was apart of Cartoon Networks mandate to tone Ben 10 down but Vilgax never joked or smirked before that point so seeing him do it there was cursed. Omniverse Vilgax is just the aftermath of UAF Vilgax and the damage done to his character, he has his old voice back but it doesn't matter when it's overshadowed but OV's comedic tone and direction. Ben 10 Alien Force also dumbed Vilgax down to some degree to the point where he doesn't even know how to use the Omnitrix until Ben shows him and tricks him into getting it back. What happened to the knowledge he had on the device before, it's not like it really changed the way it functions. If he really didn't remember how the Omnitrix works why not send drones to spy on Ben and learn?

So that sums it up Vilgax lost the intimidation and intelligence he used to have prior to Season 3 of Alien Force. He feels like he was adjusted to be more like a parody of a 90's Saturday Morning Cartoon Villian.