The game usually doesn’t even give you the option of asking a faction to become a vassal unless you’ve really got them over a barrel: they’ve done poorly in a war against you and only have a couple of provinces left, etc. In vanilla, not all factions can be made vassals regardless of how desperate they are.
The following appears to be true about vassal status:
- When a faction becomes a vassal, its diplomatic status will immediately mirror that of its parent state. Whomever you’re at war with, they’ll be at war with, etc. What this means is all the vassal’s previous wars will now be converted to peace, and all prior trade agreements cancelled. Notice that when a new war breaks out, all one’s vassals are automatically dragged in: they are not given an option, as they are if they are merely your allies.
- Note also that vassals will still expand, but not by military conquest: only into ruins. One way this can happen is if you make a faction into a vassal but they no longer have provinces. They’ll wander around until they find suitable ruins to occupy. Your permission will not be asked. Another way vassals can expand is when you (and therefore your vassal) are at war with a faction. They can independently assault, siege, and then occupy the cities of your common enemy. The fact that this is happening won't necessarily be brought to your attention.
- Interestingly, vassals are not required to remain vassals: they can secede. Usually this happens when their approval of you is low enough, such as when you take military action against a faction for which they have a high approval rating. When this happens, the diplomatic status of the seceding nation is set to “War.” Note that secession is effective immediately: they don’t have to win a war against you for it to become official. Also note that, despite the fact that if a faction is your vassal for a long time and then declare war on you, there's no special announcement screen for this! You'll just be scrolling over the strategy map one day only to discover that your former vassal now has that red X next it its settlement labels, indicating war: you are never informed of what happened.
- When one is about to lay the sword to a faction’s last city, one is given the option to “Subjugate.” This means convert them to vassal status against their will. There is technically no difference between factions so converted and factions that offered themselves up for vassal status, but remember that conversion to vassal status doesn’t convert a hostile status to a friendly one (though it helps a bit, because now you have a “treaty” with that faction). Therefore don’t be surprised if a forcibly subjugated state secedes from your control a few turns later.
- Vassal status can easily be cancelled by declaring war on the faction on the campaign map or in the diplomacy screen. This will entail no diplomatic penalties for the player unless the vassal treaty was established fewer than 8 turns ago. In that case, the player’s reliability will be adversely affected.
- Being a vassal does not automatically grant a trade agreement. That is a separate arrangement.
- Note that if you're ultimately interested in persuading a friendly nation of like people to join your confederation, granting them vassal status is probably not optimal. This is because one cannot move directly from vassal status to confederated status: one must be "un-vassaled" first, a procedure that entails a diplomatic penalty. Although the penalty is less severe if the vassal status was established fewer than eight turns ago, there is still a penalty.
- Members of your own race—or compatible races—will offer themselves for vassal status long before incompatible races will.
- When a state becomes your vassal, you acquire their view of the map (i.e., any fog of war is lifted from their provinces). This view remains even if the vassal status is cancelled.
- There is no maximum to the number of vassals one can have.
- If you're planning on converting a large number of factions to vassalage, you should do it before you defeat Archaon the Everchosen. After he is vanquished, factions are for some reason much more reluctant to become your vassals.
- If a faction is your vassal, they will pay you a small tribute every turn.
- If a faction is a vassal of another faction, you won't be able to interact with them diplomatically, with two exceptions: you will be able to gift them money and you will be able to strike trading agreements with them.
- If a faction has a vassal and then itself becomes a vassal, its former vassal remains a vassal, only switching its allegiance to the new master.
- For a faction to become eligible for vassal status, they must be in possession of at least one province. If they are merely a homeless army, you will not be given the option to demand vassalage of the homeless faction.
- If a faction you are fighting becomes a vassal in the middle of your war, you will be given a dialogue choice between making peace with the vassal or declaring war also on their new masters.
- Note that although orcs can become your vassals, if they park an army on your property, it will still be "raiding" your province and have the usual negative effects on public order, even though your orc vassal still appears as an "allied army."
- When you "Liberate" a faction, that does not create a vassal: it creates a military ally.
- The game considers vassalage to be a treaty that lasts forever and does not expire, as with all treaties. Therefore, if, after many, many turns, you set a vassal free, you still receive the same penalties as if you had broken a treaty right after making one.