How to vassalize (choice not available)?

[unmerged(1494)]

I'm playing England. Scotland is my 'best of friends' with a +199 relationship. We are not at war and have a royal marriage. I'd like to ask them to become a vassal, but the choice in the diplomacy window is dimmed. Is there another criteria for vassalization other than having a royal marriage and a very friendly relationship? They have a permanent causus belli against me and are in an alliance that I am not part of.

 

[UnionBuster]

Going from memory, but I think you must be in the same alliance.

 

[unmerged(548)]

Yes, you need to be in the same alliance.

Generally when in doubt, hover the mouse over the option and the in game help text should pop up with hopefully usefull explanation.