I Suppose you did not made venice vassal cause it is not imperial province
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Partly true. You can argue, then why not keep Zeta and the Byzantine provinces then? They are far from the empire! The main reason is intend to keep my stability cost as low as possible, avoid as many riots as possible and releasing vassals reduces your infamy significantly. I may get 10 infamy from my annexations, but once I have released vassals afterwards, I am down to 2 infamy.
In addition, vassals provides a 'buffershield'. First of all, ensure you are in alliance with your vassals. Secondly, a good relationship between you and a vassal ensures a stable relationship, but most importantly they won't betray you. Which means you can fight wars against
their neighbours without dragging them into the war, so you always have a safe location to station your armies and more importantly your enemy can never reach your provinces.
Vassals means less costs and requirements by its master. Obviously, there are a downside, in that you do not get all the benefits of their provinces, but only a percentage. In addition, you don't have direct control over them and let the AI control what they are doing. But this is far less of a concern of mine. Whatever the AI does - other than getting themselves into stupid wars - is pretty much regardless.
However, Venice is a special case. It is a rich province, and for Ulm to benefit entirely from this, they need direct control of it. And if I can only pick
one province beyond my own, Venice would be it. In addition, as you said yourself, it is not an imperial province. And more over, it is easy to protect with a small fleet. But for now, I just need to ensure I attack enemies of Austria, since they are they only ones with direct access to it, and I won't have a fleet until 1501 (core province).
Once finished, this AAR should be compiled and stickied as a tutorial. Simple, elegant, clear, smart, entertaining and yet incredibly informative. Thank you once more.
You are most welcome. And thank you for your kind words.