A vassal nation is a nation that has chosen to be subordinate to another nation, called the overlord nation. Vassals receive protection and bonus claims from their overlord nation, but sacrifice some of their independence in return.
To become a vassal nation, a nation's Leader or Coleader may request another nation to become its overlord by using /vassals pledge (name of overlord nation). The other nation's Leader or Coleaders must be online in order to accept the request by typing /accept or deny it by typing /deny. This costs diplomatic points for both the vassal and overlord.
The overlord and its vassals can communicate through the vassal chat channel, using the command /vc (message). Messages in this channel will be shared between the overlord and all of its vassals.
Vassals are permanent allies of their overlord nation. They also share the same enemies, and the vassal can only form alliances with their overlord's allies.
Nobody can declare a vassal nation as an enemy directly - they must enemy its overlord instead. If the overlord is powerful, this can deter nations from attacking the vassal.
Vassal nations inherit the nation claim bonus of their overlord, and vice versa. For vassals, the nation bonus received from the overlord depends on how much of the vassal is occupied. For example, if 40% of the vassal's towns are occupied, it will only receive 40% of the nation bonus of its overlord.
The overlord nation has several powers over their vassals.
The overlord nation's Leader/Coleader can rename a vassal using /vassals rename <name of vassal> <new name>. Vassals cannot change their names themselves.
/vassals settax (vassal name) (percentage).
A vassalage relationship can be terminated in four ways.
/vassals release <vassal nation>./vassalrevolt.
If the overlord refuses to release a vassal, it can free itself by beginning a vassal revolt using/vassalrevolt. This forcibly releases the vassal from vassalage. However, all of its towns will lose siege immunity and its unoccupied towns will become occupied by its former overlord.
If the vassal was subjugated, they can only revolt 10 days after being subjugated. The time remaining until such a vassal can revolt is shown in their /n information screen.
After revolting, your former overlord will be unable to subjugate your nation for 96 hours (4 days). During this period, you must begin a revolt or liberation siege to free your towns. Otherwise your former overlord will be able to subjugate and re-vassalize you.