I do think it's interesting that you are operating upon the assumption that a vassalized state will always desire to separate from their nominal ruler. Rather than potentially deciding that coexistence/cooperation is in their best interests after all. There are several historical examples where a group of separate states unified into a greater whole.
That being said, the system, as detailed, most certainly allows for a vassal to still expand their power, and even induce wars by inciting other players(in the case of A.I.), potentially causing their overlord to be knocked out, if played carefully(and depending on if their overlord is automatically declared war upon as well). Freeing the vassal to be an independent state once more.
As it stands, it appears that your(@Jolly Joker) primary opposition to the mechanic is within a PvP view of things(please do correct me if I'm wrong, though). You and/or others have already sufficiently detailed why it's problematic in such a system. Either a vassal is somebody that arguably could have just been a direct ally(if they're fully willing to cooperate), or they are somebody who is highly likely to message other players with the intent to sabotage their overlord as much as possible, such that they can break free.
In the former case, there is, again arguably, little reason for the system to exist, rather than simply offering an alliance(although in a direct free-for-all, alliances may not be an option, whereas vassalage may be fair game, pending 'house rules').
In the latter case, there is little reason for the system to exist, as most savvy players will simply eliminate players rather than offer vassalage. After all, if a vassal becomes your ally(with ally-vision, etc.), they might be in a greater position to damage your position, by sharing information to other players, than before when they were simply a player that was losing to you.
So, yes, ultimately, outside of house rules, and particular groups of players, the Vassal mechanic is likely to be ignored in PvP games.
But!, I don't believe that is sufficient reason to rule out the system, for multiple reasons:
1) The game is far and away mostly played in single player. This is a fact.
2) There are still people who will utilize it in multi-player.
3) In PvA.I. games, or mixed PvA.I./PvP games, I could see it being used reasonably often. Partially to tell a story(in your example of the unruly A.I. Vassal becoming docile, one could imagine that they are heavily policed or even enslaved with cybernetics, chemically or psionically), and partially for reasons stated by other posters(a loyal ally, laziness, etc.). As said by many here, it's a more robust replacement of the Surrender mechanic.
Being a primarily PvP player myself, I don't see myself using the mechanic all that often, but I see no reason why it shouldn't exist. After all, if you don't want to be a vassal, then refuse the offer of vassalage. In singleplayer, that will generally mean you start a new game, unless you can turn around the war(or reload to a point sufficiently in advance). In multiplayer, well, it's basically the same result, I suppose.
All the arguments of how it ruins playing the game if you become a vassal are a bit silly to me, personally, when you can simply refuse the offer. At least as a vassal you can still play and potentially win the game, even if it's by allied victory instead of solo victory. That, and A.I. is likely not going to decide to wipe you out after the fact(although it would be interesting).
The alternative is just likely starting a new game after refusing vassalage and being defeated(if you don't turn the war around, as mentioned)... which is what would happen if you were going to lose a war, and there was no vassal option, anyway.
That being said, the system, as detailed, most certainly allows for a vassal to still expand their power, and even induce wars by inciting other players(in the case of A.I.), potentially causing their overlord to be knocked out, if played carefully(and depending on if their overlord is automatically declared war upon as well). Freeing the vassal to be an independent state once more.
As it stands, it appears that your(@Jolly Joker) primary opposition to the mechanic is within a PvP view of things(please do correct me if I'm wrong, though). You and/or others have already sufficiently detailed why it's problematic in such a system. Either a vassal is somebody that arguably could have just been a direct ally(if they're fully willing to cooperate), or they are somebody who is highly likely to message other players with the intent to sabotage their overlord as much as possible, such that they can break free.
In the former case, there is, again arguably, little reason for the system to exist, rather than simply offering an alliance(although in a direct free-for-all, alliances may not be an option, whereas vassalage may be fair game, pending 'house rules').
In the latter case, there is little reason for the system to exist, as most savvy players will simply eliminate players rather than offer vassalage. After all, if a vassal becomes your ally(with ally-vision, etc.), they might be in a greater position to damage your position, by sharing information to other players, than before when they were simply a player that was losing to you.
So, yes, ultimately, outside of house rules, and particular groups of players, the Vassal mechanic is likely to be ignored in PvP games.
But!, I don't believe that is sufficient reason to rule out the system, for multiple reasons:
1) The game is far and away mostly played in single player. This is a fact.
2) There are still people who will utilize it in multi-player.
3) In PvA.I. games, or mixed PvA.I./PvP games, I could see it being used reasonably often. Partially to tell a story(in your example of the unruly A.I. Vassal becoming docile, one could imagine that they are heavily policed or even enslaved with cybernetics, chemically or psionically), and partially for reasons stated by other posters(a loyal ally, laziness, etc.). As said by many here, it's a more robust replacement of the Surrender mechanic.
Being a primarily PvP player myself, I don't see myself using the mechanic all that often, but I see no reason why it shouldn't exist. After all, if you don't want to be a vassal, then refuse the offer of vassalage. In singleplayer, that will generally mean you start a new game, unless you can turn around the war(or reload to a point sufficiently in advance). In multiplayer, well, it's basically the same result, I suppose.
All the arguments of how it ruins playing the game if you become a vassal are a bit silly to me, personally, when you can simply refuse the offer. At least as a vassal you can still play and potentially win the game, even if it's by allied victory instead of solo victory. That, and A.I. is likely not going to decide to wipe you out after the fact(although it would be interesting).
The alternative is just likely starting a new game after refusing vassalage and being defeated(if you don't turn the war around, as mentioned)... which is what would happen if you were going to lose a war, and there was no vassal option, anyway.
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