I started my GC campaign as Poland so I have Prussia as my vassal. However it seems I'm unable to annex it diplomatically as I dn't have a royal marriage with Prussia. Normaly having royal marriage is necessary to vassalaize the country so as Prussia is already my vassal this option in my diplomatic screen is greyed. Is this normal situation and if so, was it planned to be that way by the authors or is just a bug. Could it be that this was done to reflect the fact that Prussia being a vassal of Poland is result of a forced vassalisation - one of the conditions of the 2nd Treaty of Torun (1466).