Ben 10: Alien Force season 2 was a major improvement, in my opinion. It had much better episodes and use of characters than the first season, and managed to wrap up the DN Aliens
arc in a satisfying way (In my opinion). So with all that improvement, you would think that season 3 would be raisng the bar even higher, and hey they brought Vilgax, the series main villain back to bring his wrath to Earth. This is going to be awesome right?!
Vengeance of Vilgax does not improve on the show further, but rather sets it back. This episode embodies everything wrong about Alien Force, and is a massive disappointment. Alright, I will say this: the opening scene where Vilgax brutally murders the Galactic Enforcers is an excellent set up. It's brutal, impressive and it makes Vilgax threatening again. The main plot is that Vilgax is going to Earth to challenge Ben to MORAL KOMBAT!! for rule/destruction of Earth. It's a law that was used to stop planetary war (How that is any better is beyond me). Now naturally, this plot seems too simple. So, Ben, ego inflated by all his congratulations after the events of last season, tries to hack the Omnitrix because he has sadly gotten a case of "Teh Stupids", and winds up separating Chromastone, Way Big, Goop and Spider Monkey from his Onmitrix, and transforms Kevin's body into a stone/metal/crystal fusion.
This is all executed in a terrible fashion. The Omnitrix retrieval seems like pointless padding, and seems more like a fanservice excuse than anything (Look a the Spider Monkey and Goop fights and guess which fetish they were playing on). The fights for this part are really mediocre at best, and Gwen and Kevin have been turned into Worf clones.
But the biggest problem is Vilgax. Simply put, he is nothing, NOTHING, like the original character. He has a different personality, different abilities and a completely different appearence. He is now a crappy Darkseid clone (He blatantly copies Darkseids' Omega Beams) with some Sylar thrown in for about 2 minutes. And he contributes nothing. You could easily replace Vilgax with a Monster of the Week with the same agenda, and this episode would feel no different.
I could go on, but I think I will finish by saying that Vengeance of Vilgax is a huge failure. The new season will have a lot to make up for.