Capitulated vassels work on the basis that they surrender to there master, they can break from there master only should there master become to weak to hold influence over them, currently if there land/pop etc go over/under 50% they can potencially break away, this is the only way for them to break away from a forced vassaledge.
If the master capitulates to another player then he is clearly to weak to continue to hold sway over a forced vassel and it's quite correct they be released, such a vassel should not automatically gain a peace treaty with the player, they should however always agree to one should the player want it, since a civ who has capitulated to another civ should not want to keep a war going vs someone who has just taken down his master since his master had them under vassaledge through force of arms, and your force of arms is clearly better otherwise you would not have conquered the master, so such a vassel should accept peace without hesitation, if the player offers a treaty.
For a mutual vassaledge (one done by consent from both parties), the other civ should not automatically want to sign a treaty as above, but should do so based on the standard civ modifiers of if a civ accepts peace or not, perhaps a positive modifier should be spun in taking into account it's no longer a war of 2 vs 1 so clearly the war now a more serious one for the other civ, but consensual vassels should make there own descisions on war or peace with you after the master civ falls, captitulated one's should seek peace but not be guranteed it (player choice)
so yeah agree with current implementation although if it does'nt include the capitulated vassel of a fallen master seeking peace as a preferance to continued war then i think it should be added for logic reasons