Hi. 

• Summary: The game breaks down when an enemy becomes a vassal to someone else while I am sieging them.
• Description: While sieging Babylon, it seems they became a vassal to the Huns. I think I was allied with the huns, but either way it put me at peace with Babylon (now the Aztecs). But I still have them sieged. I attack the next turn, win, occupy the city, but because I am at peace, all diplomacy screens are bugged out. I don't believe there is a way forward.
• Steps to Reproduce: Siege a city. Make that culture become the vassal to a third person. Attack the city and occupy it. Game doesn't know what to do.
• Reproducibility: ?
• Expected Behavior: I should remain at war with Babylon (Aztecs) despite vassalization, or they should not be able to become a vassal while I have their capital sieged. Or, there should be a prompt, to continue with the siege will put me at war with the Huns. Something like that.
• Notes: I love the game and commend the team for making a game that looks fantastic and plays great. The combat is so much fun. The AI has put up good fights, especially on the defense. Thank you for the opportunity to play this opendev. I look forward to the full game.auto-save 144.ctr

I came across almost the same issue.

I'm at war against yellow empire, sieging Londonia (north of my capital), and after vassalizing the AI, the siege continue and blocks me from ending turn, AI won't move on their battle turn.

Savefile, logs and diags attached.

BlockedInSiegeAfterVassalization_126.ctr

Diagnostics (2021'05'03 @1558'49''559''')p21.html

Player.log