The AI's gotten kind of upset. Somewhere along the way it decided you puny humans need more interesting ways to lose (or win, if you're one of those people, but who are we to judge). Vengeance of the Machine offers new tools to the player (like Core Turret Controllers) and a new frontal-confrontation victory condition (Showdown Devices), but focuses on enhancing the already-incredible variety of your main opponent: the AI. The possibilities grow exponentially with the addition of the new "Dual Type AI" system (ever wonder what a single AI player would be like with Thief AND Speed-Racer? Find out!), AI Planetary Subcommanders, and Dire Guardians. They say the best stories revolve around an awesome villain, and this expansion takes AI War's villainy to new heights.
For the exhaustive list of features, please see this page on the wiki.
8 new AI types, from the honorable "Chivalric" which refuses to harass your Advanced Factories and other irreplaceables to the bleedingly-hard "Vicious Exotic" which eschews normal waves in favor of hammering you with massive Exogalactic Attacks.
2 new Map styles: Wheel and Honeycomb.
6 new Bonus Ship Types, from the modular Protector Starship to the hilarious "space dock artillery cannon" Neinzul Railpod.
2 new optional AI Plots: Hunter (which brings the AI's Special Forces from strong to nightmarish) and Shark.
5 new Core Guard Posts to defend the AI homeworlds.
7 new Guardian types to add variety to the AI's heavier units.