I think I've figured it out: A capped vassal asking you to stop trading with his master. Not realistic (should result in the master going to war against their vassal again, if you think about it. The vassal is trying to organize an embargo against him!), and there is no meaningful choice thanks to the blended stance/diplomacy aspect of master and vassals. It boils down to "piss us off or piss us off." What a choice.
Anyways, room for argument on the most annoying thing, but that's my pick.
Unless you wish to keep the vassal's attitude up high so they will trade with you, you can just ignore everything they want from you. Vassals have no war status of their own and nothing you do to them diplomatically reflects on the master (unless, of course, you declare war/bribe war on both of them). You can make all the demands or refuse as many of theirs as you want, if you don't care about trading techs with a (likely) weaker and backwards AI that got conquered. There are exceptions, especially when the peace vassal mechanic is in play, where the little vassal is quite advanced like Gandhi/Huayna/Pacal etc but I usually focus on the bigger fish and taking it down first.
My personal gripes:
-general unit selection issues, most of them pertaining to groups and mixed stacks
-Peace vassaling in general is very advantageous for the AI due to trading rules, and the war-status rules. It is an endless headache on maps with lots of AI size disparity, where the little turds will sell themselves to a master in response to rising military powers, or my
favorite, when a master accepts the vassal as war bribe just as the little horsehocky is getting his ass kicked by me, starting another war immediately and denying me the capitulation (which is why I have moved to just killing off AIs ASAP instead of fishing for caps lately, out of spite). Many reloads have been had from forgetting to check that small detail before DoWs, though it is usually able to be countered. It's poorly balanced as well because even though you can accept Peace Vassals, you benefit very little from it compared to just having the AI be Friendly/Pleased anyway. And 9/10 times I'd rather have their land or their cities than directing their research, as by the point they start doing this (It's triggered by power ratios) I'm already teched and am powering for an attack already.
-the UI buttons shifting their locations based on the availability of an action for that unit. Several times I have been just clicking through units later in the game like workers that pop up on the "needs orders" rotation only to have the spot where the "Automate" button change to something like "Fort" or have been trying to group units for a "Heal" or "Move to" order only to accidentally hit "Explore" and send the whole stack away!
-Units executing their orders before I can stop/reorder them because the game forced me to give an order to a different unit first, and doing ANYTHING other than scrolling to the unit I want to stop and ordering it to can set off the entire queued action stack (and sometimes still does after ordering the first unit on a tile, the forced shift-back will pull the trigger on the queue stack). I've had to go back 2 turns to prevent mistimed chops before. I looked into BULL but I honestly didn't want the extra info and overflow gold changes, it feels like too much.
-The AI's cheating vision through fog of war to snag unprotected workers/cities or kill a weak unit from so far away you could never expect it because, well, YOU CAN"T SEE THEIR UNITS IN THEIR TERRITORY. Extremely frustrating. A very common scenario is for them to zip down the road network from some spot 6 tiles away the turn after you take a city in a culture contested area before your culture expands to slow their movement and the lack of a city to defend frees up their nearby units somewhat. It makes me so glad that the AI cannot capture Settlers/Workers unless they are at war already. In Civ Rev they would just take your Settler/Spy no questions asked.
-A similar gripe about the fog of war, trying to give units a distance move order only to find out the AI (clarification: at war with this AI) decided to plant a unit RIGHT THERE in the projected path, under the cover of Fog of War at the spot and from wherever it moved, ruining or wasting the movement. I learned from Civ Rev to never trust waypoint pathing and move units one tile at a time, but in Civ4 mid/late game the volumes of units in play makes it unfeasible to always move one unit form each group ahead on the path to kill off a roadblock EVERY TIME I want to move a stack across the map. I regularly produce and send upwards of 400-600 war units around the map over the course of some Earth18 games, it's seriously annoying.
-hating on the combat odds is very common around here, I'm no exception. What really pisses me off about the way they roll is how
ridiculous the disparity in outcomes can be. Game I'm playing right now, Izzy settles in my face, I had BFC copper, so I Axe her. Her last city is 3 archers, one fresh (no fortify), hill, no promos, 20% culture vs. 7 axes (4 of them CR1/2, one medic, one no promo, and one Cover) +2 Warriors. First try: wiped out to a man, only one archer killed. Reload: take the city losing one Axe. What. the. Hell. THAT is why I do reload early game fights because crap like this is so extreme one way or another. I had fully planned to just eat a loss that shaved an archer or two while queuing up more axes and waiting (hence the medic) as I expected 4 archers instead of 3, I had 2 more axes inbound for a total of 9 axes coming and was ready to produce 2 more since she was over 10 tiles away. Instead I turned them around and emptied the queues. I have a fair amount of experience in choking and generally wearing down AIs over time (I tore her down from 4 cities) but if it's going to be so coin flip I'd be better off creating one attacker per defender and just scum each fight, jeez. Or never even bothering to try to attack her, to the other extreme.