The vassal rule has been changed. It is now
2. Vassalship between humans.
a) As a vassal you are not allowed to use the public diplomatic actions (i.e. those that can be set to be seen by your master in the game log) in a way that contravenes the interest of your master (e.g. MA, embargoes, alliances, etc, etc), regardless if you are the sender or receiver of the diplomatic action; you may however talk as much as you want - you can thus plot vs him and reveal his plans etc - and you may do those actions that cannot be seen by your master, like giving or sending a state gift)
b) Both parties must protect and support eachother when attacked (as good as they can, this is arguably a questionable rule since it is impossible to define accurately)
c) The master may cancel the vassalship any time he wants but the vassal cannot do it until 20 years have passed (both can do it merely by cancelling it)
2. Vassalship between humans.
a) As a vassal you are not allowed to use the public diplomatic actions (i.e. those that can be set to be seen by your master in the game log) in a way that contravenes the interest of your master (e.g. MA, embargoes, alliances, etc, etc), regardless if you are the sender or receiver of the diplomatic action; you may however talk as much as you want - you can thus plot vs him and reveal his plans etc - and you may do those actions that cannot be seen by your master, like giving or sending a state gift)
b) Both parties must protect and support eachother when attacked (as good as they can, this is arguably a questionable rule since it is impossible to define accurately)
c) The master may cancel the vassalship any time he wants but the vassal cannot do it until 20 years have passed (both can do it merely by cancelling it)