I always have a soft spot for stories, and TV programmes and films in general, that have the main antagonist and protagonist join up and fight what they both no realize is their common enemy. Darth Vader and Luke Skywalker teaming up briefly to dispose of the Emperor in Return of the Jedi is an example that leaps immediately to mind. Tobias Vaughn has been such a great character during the first seven episodes of this story that I was really hoping he wouldn't be wasted in the final installment once his involvement in the Cybermen's plans came to an end. And, thankfully, he joins up with The Doctor to exact revenge on his former allies.
He and The Doctor make a formidable, if short-lived, team. I still think it's a shame that Vaughn dies. It would have been much more interesting, in my opinion, if Vaughn would have escaped after helping The Doctor disable the Cybermen's radio signal, thus sign posting a potential reunion between the pair down the road (which never would have been made, if the show's producers knew what was good for them. Leave the fate of Vaughn to the fan fiction world). As it happens, though, Vaughn is gunned down in long shot by the Cybermen in a bit of an anticlimactic end to one of the best villains in Doctor Who history.
There are so many elements competing with each other throughout the eight episodes of The Invasion. The Doctor, Jamie, and Zoe, Isobel and Professor Watkins, Tobias Vaughn, Packer, and so on. The most surprising element of this story is the almost complete absence of the Cybermen. They only feature in three of the eight episodes, and they are often reduced to unintelligent drones doing the bidding of the Cyber Director or Vaughn and his minions. In retrospect, after their meagre impact in this episode (barring one or two notable scenes, of course), as well as the relative failure that was The Wheel in Space (and the perceived substandard nature of their next adventure, 1975's Revenge of the Cybermen), one could argue that it took 15 years for the Cybermen to regain their glory (with 1982's Earthshock) after the smashing The Tomb of the Cybermen.
Another, perhaps equally surprising as the Cybermen being ignored, is how little a role, at times, The Doctor, Zoe, and Jamie play in this story. Zoe solves a problem regarding missiles in Episode Seven, but is otherwise often in the background. Jamie, during the second half of the story, at least, is quite ineffectual, sleeping for most of his time after the sewer rescue, and not present at all during Episode Eight (apart from the final leaving scene). The Doctor does manage to get close to disabling the radio signal midway through Episode Eight, but he needs UNIT's help to finish the job. Once that is done, he's reduced to a bystander while UNIT, the real focus of this story, destroys both the Cybermen's missiles and the Cybermen's mother ship. This is the most powerful we will ever see UNIT. In the future, they will try and fail, and will require and seek The Doctor's help - at first, resentfully; later, with more enthusiasm. Here, The Doctor agrees with everything that UNIT is planning, guns blazing and all. It's a promising outlook, and one that paints UNIT in a remarkably positive light, but isn't actually the "promise of the future" for the Pertwee era that it is often made out to be.