Vengeance on Varos is a great improvement on the previous episodes, and even approaches sophistication.
It feels, like other stories of this era, like people are having a science fiction adventure show that the Doctor wanders into by accident. For a change it feels like the ending couldn't have happened without the Doctor. In fact the Doctor and Peri don't appear on the planet until what would be the cliffhanger for the 1st 25-minute episode (it was written and broadcast as 2 45-minute episodes). It's a fun little story, with Sil and with the people (Arak and Etta) watching the executions on television complaining that they were repeats. It's very metatextual, even by modern standards. The cliffhanger for the first episode is someone saying "And cut it... Now!" and someone cutting off the live feed of the Doctor, and then it goes to the theme music maybe a second later. The CCTV viewers commenting on the plot and the Doctor's coat, etc, the lines "Do you always get the priest parts?" and [nearly repeated from the last story] "All these corridors look the same to me, Doctor!", etc... And then there's a TV show which had been criticised for the levels of violence, exploring that very idea! Even the Doctor himself seems desensitised: doesn't appear to regret any of the [arguable accidental] deaths he caused.
There are dodgy bits - the expository dialog is often bad, and Arak and Etta doing nothing but effectively sitting around watching Doctor Who all episode doesn't exactly sell the idea that they are the populace of the planet are worked as hard as slaves. The Doctor's depressive episode at the start seems to have little function other than to delay his arrival into the plot while making sure that he's there with a reason, and the basic premise that the mineral is not merely valuable but so rare that only one planet known to Gallifrey is the source of it, but that another gets immediately discovered as soon as it becomes convenient to the plot - shouldn't have survived the script edit untweaked (and it's not a big tweak needed, just have it be the only source within range of where the TARDIS stopped). But then there are good human drama bits, like the Governor's pleading to Maldak, and the amusing but rather touching bit at the very end:
Arak: No more executions, torture, nothing!
Etta: It's all changed! We're free!
Arak: Are we?
Arak: What shall we do?
Etta: (beat) Dunno...
[cut to static, and end]
This is not Doctor Who firing on all cylinders but it knows what that looks like, which it hasn't done for a while. How different from the last two.