That's absurd. I wasn't trying to wipe them out, I was trying to make them my vassals. Just showing up with my navy should have scared them into surrender, I had like 10x the strength of them. Certainly blowing up every last one of their warships, science vessels, and construction ships should have made their decision easy. Certainly the fact that I occupied their capital was something they should have been worried about... They should have been thrilled I was offering both my protection and access to my fantastic tech... Instead the AI insisted on being completely stomped, which obviously puts them further from victory.
Hmm, somethow this get me thinking of Afganistan. 2 superpowers have in turn wiped out all visible opposition, but neither been able to claim it as 'vassal'.
It can always be discussed how to model it in games, but I find it fair, that winning 'Hearts and Mind' / having a true Vassal takes a lot longer than just wiping out the opposing assets.
There are some important differences here. I think Iraq is the better metaphor, but both probably work.
Superpower(s) show up and dismantle the existing problematic state. The problematic state pretty much instantly surrenders, because people don't want to be dead. How long exactly was iraq's military a problem during the invasion? It wasn't at all.
The next step is what you're talking about. OK, now I'm nominally in charge of iraq. The state I fought is gone. But, the pops are unhappy! They don't like my ethics! Oh no! Unrest!
This is already modeled in game with fully conquered planets, but not with vassals. You take over a bunch of planets, but the pops won't do any work or produce anything until you make them happy, "Winning hearts and minds".
Having the pops of an empire that agrees to be a vassal start revolting would make some sense, or even having parts of that empire splinter off into separate new empires. What doesn't make sense is the state whose ass I just kicked being the one who won't surrender. Like... I'm here with a gun to your head, no isn't an option dummy. You think Saddam wouldn't have agreed to pretty much anything to stay in charge of Iraq?
Basically, you storm the capital of a state, you have the leader of the state at your mercy, they surrender, because why the hell wouldn't they? Don't they want to keep living, and potentially keep whatever power they have and maybe use that to defeat me someday?
Since we're talking about aliens, it'd be ok if the some of the super fierce militant aliens really were willing to fight to the last soldier, but that should be the exception.