If using BUG mod, hover over the vassal on the scoreboard
-if you are given info like "% of original population/land area, free at %" that vassal is a capitulate. They will only break free when either they get BOTH 50% or more of their master's current land and pop, or when they get knocked below 50% of the size they were (vassal) when vassaled...and when they want to. Capitulates can refuse to break free if they have a good diplomatic relationship with the master or are considerably weaker than both their master and attacker when at war.
-if all it says is 'Vassal of" and nothing else, that vassal is a peace-vassal. Peace-vassals can freely dissolve the arrangement, but will often refuse to if they are in a good relationship with the master diplomatically unless pushed to break off by having a war declared on them both. Peace-vassals can and will also re-vassal immediately after breaking away, especially if they are weak, and sometimes even to another party entirely. This CAN happen while you are at war with them, so mind it when looking at who might be bribed against you in a war you declare against the original pairing....you may suddenly have a new suzerain DoWing you!
-If you check the diplomacy screen and see a "you granted us our independence" modifier toward the master, that vassal is a liberated colony, technically a peace-vassal. As far as I know colonies never break free in standard BTS due to the massive +10 diplo modifier, or some hard coded reason...I have always, ALWAYS joint-capitulated them Master>then vassal on same turn or killed the master entirely before vassal would give in.